diff --git a/common/uipriv.h b/common/uipriv.h
index 8bffda4e..a36be4eb 100644
--- a/common/uipriv.h
+++ b/common/uipriv.h
@@ -12,10 +12,12 @@ extern void *uiAlloc(size_t, const char *);
 extern void *uiRealloc(void *, size_t, const char *);
 extern void uiFree(void *);
 
+#define _ns2(s) #s
+#define _ns(s) _ns2(s)
 extern void _implbug(const char *file, const char *line, const char *func, const char *format, ...);
-#define implbug(...) _implbug(__FILE__, #__LINE__, __func__, __VA_ARGS__)
+#define implbug(...) _implbug(__FILE__, _ns(__LINE__), __func__, __VA_ARGS__)
 extern void _userbug(const char *file, const char *line, const char *func, const char *format, ...);
-#define userbug(...) _implbug(__FILE__, #__LINE__, __func__, __VA_ARGS__)
+#define userbug(...) _implbug(__FILE__, _ns(__LINE__), __func__, __VA_ARGS__)
 
 // control.c
 extern uiControl *newControl(size_t size, uint32_t OSsig, uint32_t typesig, const char *typenamestr);
diff --git a/darwin/GNUfiles.mk b/darwin/GNUfiles.mk
index 49acd1d4..85d89615 100644
--- a/darwin/GNUfiles.mk
+++ b/darwin/GNUfiles.mk
@@ -13,6 +13,7 @@ MFILES += \
 	darwin/datetimepicker.m \
 	darwin/draw.m \
 	darwin/drawtext.m \
+	darwin/debug.m \
 	darwin/entry.m \
 	darwin/fontbutton.m \
 	darwin/group.m \
diff --git a/darwin/alloc.m b/darwin/alloc.m
index 5fd42f3b..9f0531ba 100644
--- a/darwin/alloc.m
+++ b/darwin/alloc.m
@@ -22,7 +22,9 @@ void initAlloc(void)
 
 void uninitAlloc(void)
 {
+	NSMutableString *str;
 	NSUInteger i;
+	NSValue *v;
 
 	// delegates might have mapTables allocated
 	// TODO verify they are empty
@@ -33,16 +35,14 @@ void uninitAlloc(void)
 		[allocations release];
 		return;
 	}
-	fprintf(stderr, "[libui] leaked allocations:\n");
-	[allocations enumerateObjectsUsingBlock:^(id obj, NSUInteger index, BOOL *stop) {
-		NSValue *v;
+	str = [NSMutableString new];
+	for (v in allocations) {
 		void *ptr;
 
-		v = (NSValue *) obj;
 		ptr = [v pointerValue];
-		fprintf(stderr, "[libui] %p %s\n", ptr, *TYPE(ptr));
-	}];
-	complain("either you left something around or there's a bug in libui");
+		[str appendString:[NSString stringWithFormat:@"%p %s\n", ptr, *TYPE(ptr)]];
+	}
+	userbug("Some data was leaked; either you left a uiControl lying around or there's a bug in libui itself. Leaked data:\n%s", [str UTF8String]);
 }
 
 void *uiAlloc(size_t size, const char *type)
@@ -86,7 +86,7 @@ void *uiRealloc(void *p, size_t new, const char *type)
 void uiFree(void *p)
 {
 	if (p == NULL)
-		complain("attempt to uiFree(NULL); there's a bug somewhere");
+		implbug("attempt to uiFree(NULL)");
 	p = BASE(p);
 	free(p);
 	[allocations removeObject:[NSValue valueWithPointer:p]];
diff --git a/darwin/area.m b/darwin/area.m
index 72aee13c..ada4ae6d 100644
--- a/darwin/area.m
+++ b/darwin/area.m
@@ -335,7 +335,7 @@ int sendAreaEvents(NSEvent *e)
 void uiAreaSetSize(uiArea *a, intmax_t width, intmax_t height)
 {
 	if (!a->scrolling)
-		complain("attempt to call uiAreaSetSize() on a non-scrolling uiArea");
+		userbug("You cannot call uiAreaSetSize() on a non-scrolling uiArea. (uiArea: %p)", a);
 	[a->area setFrameSize:NSMakeSize(width, height)];
 }
 
@@ -347,7 +347,7 @@ void uiAreaQueueRedrawAll(uiArea *a)
 void uiAreaScrollTo(uiArea *a, double x, double y, double width, double height)
 {
 	if (!a->scrolling)
-		complain("attempt to call uiAreaScrollTo() on a non-scrolling uiArea");
+		userbug("You cannot call uiAreaScrollTo() on a non-scrolling uiArea. (uiArea: %p)", a);
 	[a->area scrollRectToVisible:NSMakeRect(x, y, width, height)];
 	// don't worry about the return value; it just says whether scrolling was needed
 }
diff --git a/darwin/debug.m b/darwin/debug.m
new file mode 100644
index 00000000..ec55a84b
--- /dev/null
+++ b/darwin/debug.m
@@ -0,0 +1,37 @@
+// 13 may 2016
+#import "uipriv_darwin.h"
+
+// TODO don't halt on release builds
+
+static void bug(const char *file, const char *line, const char *func, const char *prefix, const char *format, va_list ap)
+{
+	NSMutableString *str;
+	NSString *formatted;
+
+	str = [NSMutableString new];
+	[str appendString:[NSString stringWithFormat:@"[libui] %s:%s:%s %s", file, line, func, prefix]];
+	formatted = [[NSString alloc] initWithFormat:[NSString stringWithUTF8String:format] arguments:ap];
+	[str appendString:formatted];
+	[formatted release];
+	NSLog(@"%@", str);
+	[str release];
+	__builtin_trap();
+}
+
+void _implbug(const char *file, const char *line, const char *func, const char *format, ...)
+{
+	va_list ap;
+
+	va_start(ap, format);
+	bug(file, line, func, "POSSIBLE IMPLEMENTATION BUG; CONTACT ANDLABS:\n", format, ap);
+	va_end(ap);
+}
+
+void _userbug(const char *file, const char *line, const char *func, const char *format, ...)
+{
+	va_list ap;
+
+	va_start(ap, format);
+	bug(file, line, func, "You have a bug: ", format, ap);
+	va_end(ap);
+}
diff --git a/darwin/draw.m b/darwin/draw.m
index a147283c..6661d6a1 100644
--- a/darwin/draw.m
+++ b/darwin/draw.m
@@ -26,7 +26,7 @@ void uiDrawFreePath(uiDrawPath *p)
 void uiDrawPathNewFigure(uiDrawPath *p, double x, double y)
 {
 	if (p->ended)
-		complain("attempt to add figure to ended path in uiDrawPathNewFigure()");
+		userbug("You cannot call uiDrawPathNewFigure() on a uiDrawPath that has already been ended. (path; %p)", p);
 	CGPathMoveToPoint(p->path, NULL, x, y);
 }
 
@@ -36,7 +36,7 @@ void uiDrawPathNewFigureWithArc(uiDrawPath *p, double xCenter, double yCenter, d
 	double startx, starty;
 
 	if (p->ended)
-		complain("attempt to add figure to ended path in uiDrawPathNewFigureWithArc()");
+		userbug("You cannot call uiDrawPathNewFigureWithArc() on a uiDrawPath that has already been ended. (path; %p)", p);
 	sinStart = sin(startAngle);
 	cosStart = cos(startAngle);
 	startx = xCenter + radius * cosStart;
@@ -47,8 +47,9 @@ void uiDrawPathNewFigureWithArc(uiDrawPath *p, double xCenter, double yCenter, d
 
 void uiDrawPathLineTo(uiDrawPath *p, double x, double y)
 {
+	// TODO refine this to require being in a path
 	if (p->ended)
-		complain("attempt to add line to ended path in uiDrawPathLineTo()");
+		implbug("attempt to add line to ended path in uiDrawPathLineTo()");
 	CGPathAddLineToPoint(p->path, NULL, x, y);
 }
 
@@ -56,8 +57,9 @@ void uiDrawPathArcTo(uiDrawPath *p, double xCenter, double yCenter, double radiu
 {
 	bool cw;
 
+	// TODO likewise
 	if (p->ended)
-		complain("attempt to add arc to ended path in uiDrawPathArcTo()");
+		implbug("attempt to add arc to ended path in uiDrawPathArcTo()");
 	if (sweep > 2 * M_PI)
 		sweep = 2 * M_PI;
 	cw = false;
@@ -72,8 +74,9 @@ void uiDrawPathArcTo(uiDrawPath *p, double xCenter, double yCenter, double radiu
 
 void uiDrawPathBezierTo(uiDrawPath *p, double c1x, double c1y, double c2x, double c2y, double endX, double endY)
 {
+	// TODO likewise
 	if (p->ended)
-		complain("attempt to add bezier to ended path in uiDrawPathBezierTo()");
+		implbug("attempt to add bezier to ended path in uiDrawPathBezierTo()");
 	CGPathAddCurveToPoint(p->path, NULL,
 		c1x, c1y,
 		c2x, c2y,
@@ -82,15 +85,16 @@ void uiDrawPathBezierTo(uiDrawPath *p, double c1x, double c1y, double c2x, doubl
 
 void uiDrawPathCloseFigure(uiDrawPath *p)
 {
+	// TODO likewise
 	if (p->ended)
-		complain("attempt to close figure of ended path in uiDrawPathCloseFigure()");
+		implbug("attempt to close figure of ended path in uiDrawPathCloseFigure()");
 	CGPathCloseSubpath(p->path);
 }
 
 void uiDrawPathAddRectangle(uiDrawPath *p, double x, double y, double width, double height)
 {
 	if (p->ended)
-		complain("attempt to add rectangle to ended path in uiDrawPathAddRectangle()");
+		userbug("You cannot call uiDrawPathAddRectangle() on a uiDrawPath that has already been ended. (path; %p)", p);
 	CGPathAddRect(p->path, NULL, CGRectMake(x, y, width, height));
 }
 
@@ -132,7 +136,7 @@ void uiDrawStroke(uiDrawContext *c, uiDrawPath *path, uiDrawBrush *b, uiDrawStro
 	uiDrawPath p2;
 
 	if (!path->ended)
-		complain("path not ended in uiDrawStroke()");
+		userbug("You cannot call uiDrawStroke() on a uiDrawPath that has not been ended. (path: %p)", path);
 
 	switch (p->Cap) {
 	case uiDrawLineCapFlat:
@@ -275,7 +279,7 @@ static void fillGradient(CGContextRef ctxt, uiDrawPath *p, uiDrawBrush *b)
 void uiDrawFill(uiDrawContext *c, uiDrawPath *path, uiDrawBrush *b)
 {
 	if (!path->ended)
-		complain("path not ended in uiDrawFill()");
+		userbug("You cannot call uiDrawStroke() on a uiDrawPath that has not been ended. (path: %p)", path);
 	CGContextAddPath(c->c, (CGPathRef) (path->path));
 	switch (b->Type) {
 	case uiDrawBrushTypeSolid:
@@ -289,7 +293,7 @@ void uiDrawFill(uiDrawContext *c, uiDrawPath *path, uiDrawBrush *b)
 		// TODO
 		return;
 	}
-	complain("unknown brush type %d in uiDrawFill()", b->Type);
+	userbug("Unknown brush type %d passed to uiDrawFill().", b->Type);
 }
 
 static void m2c(uiDrawMatrix *m, CGAffineTransform *c)
@@ -421,7 +425,7 @@ void uiDrawTransform(uiDrawContext *c, uiDrawMatrix *m)
 void uiDrawClip(uiDrawContext *c, uiDrawPath *path)
 {
 	if (!path->ended)
-		complain("path not ended in uiDrawClip()");
+		userbug("You cannot call uiDrawCilp() on a uiDrawPath that has not been ended. (path: %p)", path);
 	CGContextAddPath(c->c, (CGPathRef) (path->path));
 	switch (path->fillMode) {
 	case uiDrawFillModeWinding:
diff --git a/darwin/drawtext.m b/darwin/drawtext.m
index 86fcb753..dbbe4db6 100644
--- a/darwin/drawtext.m
+++ b/darwin/drawtext.m
@@ -1,6 +1,9 @@
 // 6 september 2015
 #import "uipriv_darwin.h"
 
+// TODO
+#define complain(...) implbug(__VA_ARGS__)
+
 // TODO for all relevant routines, make sure we are freeing memory correctly
 // TODO make sure allocation failures throw exceptions?
 struct uiDrawFontFamilies {
@@ -15,7 +18,7 @@ uiDrawFontFamilies *uiDrawListFontFamilies(void)
 	// TODO is there a way to get an error reason?
 	ff->fonts = CTFontManagerCopyAvailableFontFamilyNames();
 	if (ff->fonts == NULL)
-		complain("error getting available font names (no reason specified)");
+		implbug("error getting available font names (no reason specified) (TODO)");
 	return ff;
 }
 
diff --git a/darwin/main.m b/darwin/main.m
index 07db4459..e40c0616 100644
--- a/darwin/main.m
+++ b/darwin/main.m
@@ -48,7 +48,7 @@ static BOOL canQuit = NO;
 	NSEvent *e;
 
 	if (!canQuit)
-		complain("call to [NSApp terminate:] when not ready to terminate");
+		implbug("call to [NSApp terminate:] when not ready to terminate; definitely contact andlabs");
 
 	[realNSApp() stop:realNSApp()];
 	// stop: won't register until another event has passed; let's synthesize one
diff --git a/darwin/map.m b/darwin/map.m
index 9fa2de96..67b5edb6 100644
--- a/darwin/map.m
+++ b/darwin/map.m
@@ -22,7 +22,7 @@ struct mapTable *newMap(void)
 void mapDestroy(struct mapTable *m)
 {
 	if ([m->m count] != 0)
-		complain("attempt to destroy map with items inside; did you forget to deallocate something?");
+		implbug("attempt to destroy map with items inside");
 	[m->m release];
 	uiFree(m);
 }
diff --git a/darwin/menu.m b/darwin/menu.m
index 49bf2cdc..0f33df1d 100644
--- a/darwin/menu.m
+++ b/darwin/menu.m
@@ -69,17 +69,17 @@ enum {
 	switch (smi->type) {
 	case typeQuit:
 		if (self->hasQuit)
-			complain("attempt to add multiple Quit menu items");
+			userbug("You can't have multiple Quit menu items in one program.");
 		self->hasQuit = YES;
 		break;
 	case typePreferences:
 		if (self->hasPreferences)
-			complain("attempt to add multiple Preferences menu items");
+			userbug("You can't have multiple Preferences menu items in one program.");
 		self->hasPreferences = YES;
 		break;
 	case typeAbout:
 		if (self->hasAbout)
-			complain("attempt to add multiple About menu items");
+			userbug("You can't have multiple About menu items in one program.");
 		self->hasAbout = YES;
 		break;
 	}
@@ -200,7 +200,7 @@ void uiMenuItemDisable(uiMenuItem *item)
 void uiMenuItemOnClicked(uiMenuItem *item, void (*f)(uiMenuItem *, uiWindow *, void *), void *data)
 {
 	if (item->type == typeQuit)
-		complain("attempt to call uiMenuItemOnClicked() on a Quit item; use uiOnShouldQuit() instead");
+		userbug("You can't call uiMenuItemOnClicked() on a Quit item; use uiOnShouldQuit() instead.");
 	item->onClicked = f;
 	item->onClickedData = data;
 }
@@ -225,7 +225,7 @@ static uiMenuItem *newItem(uiMenu *m, int type, const char *name)
 	uiMenuItem *item;
 
 	if (menusFinalized)
-		complain("attempt to create a new menu item after menus have been finalized");
+		userbug("You can't create a new menu item after menus have been finalized.");
 
 	item = uiNew(uiMenuItem);
 
@@ -297,7 +297,7 @@ uiMenu *uiNewMenu(const char *name)
 	uiMenu *m;
 
 	if (menusFinalized)
-		complain("attempt to create a new menu after menus have been finalized");
+		userbug("You can't create a new menu after menus have been finalized.");
 	if (menus == nil)
 		menus = [NSMutableArray new];
 
diff --git a/darwin/progressbar.m b/darwin/progressbar.m
index bb64ab2e..d8c0b060 100644
--- a/darwin/progressbar.m
+++ b/darwin/progressbar.m
@@ -11,7 +11,7 @@ uiDarwinControlAllDefaults(uiProgressBar, pi)
 void uiProgressBarSetValue(uiProgressBar *p, int value)
 {
 	if (value < 0 || value > 100)
-		complain("value %d out of range in progressbarSetValue()", value);
+		userbug("Value %d out of range for a uiProgressBar.", value);
 	// on 10.8 there's an animation when the progress bar increases, just like with Aero
 	if (value == 100) {
 		[p->pi setMaxValue:101];
diff --git a/darwin/spinbox.m b/darwin/spinbox.m
index 409bd5e0..5817efae 100644
--- a/darwin/spinbox.m
+++ b/darwin/spinbox.m
@@ -183,8 +183,9 @@ uiSpinbox *uiNewSpinbox(intmax_t min, intmax_t max)
 {
 	uiSpinbox *s;
 
+	// TODO implicitly swap instead?
 	if (min >= max)
-		complain("error: min >= max in uiNewSpinbox()");
+		userbug("min >= max is invalid for a uiSpinbox.");
 
 	uiDarwinNewControl(uiSpinbox, s);
 
diff --git a/darwin/util.m b/darwin/util.m
index fa6f5994..7031e9f8 100644
--- a/darwin/util.m
+++ b/darwin/util.m
@@ -13,15 +13,3 @@ void disableAutocorrect(NSTextView *tv)
 	[tv setAutomaticLinkDetectionEnabled:NO];
 	[tv setSmartInsertDeleteEnabled:NO];
 }
-
-void complain(const char *fmt, ...)
-{
-	va_list ap;
-
-	va_start(ap, fmt);
-	fprintf(stderr, "[libui] ");
-	vfprintf(stderr, fmt, ap);
-	fprintf(stderr, "\n");
-	va_end(ap);
-	abort();
-}