diff --git a/ai.c b/ai.c
--- a/ai.c
+++ b/ai.c
@@ -3429,7 +3429,7 @@ static void AiStateBuildDefaultAirportBl
 				MarkTileDirtyByTile(aib->use_tile);
 			}
 #endif
-//			redsq_debug(aib->use_tile);
+//			SetRedErrorSquare(aib->use_tile);
 
 			if (rule == -1) {
 				// cannot build, terraform after a while
diff --git a/sdl.c b/sdl.c
--- a/sdl.c
+++ b/sdl.c
@@ -686,25 +686,4 @@ const HalSoundDriver _sdl_sound_driver =
 	SdlSoundStop,
 };
 
-
-#if 0 /* XXX what the heck is that? */
-#include "viewport.h"
-void redsq_debug(int tile)
-{
-	_thd.redsq = tile;
-	MarkWholeScreenDirty();
-	_screen.dst_ptr = _sdl_screen->pixels;
-	UpdateWindows();
-
-	SdlVideoMakeDirty(0,0,_screen.width,_screen.height);
-	DrawSurfaceToScreen();
-}
-
-static void DbgRedraw()
-{
-	SdlVideoMakeDirty(0,0,_screen.width,_screen.height);
-	DrawSurfaceToScreen();
-}
-#endif
-
 #endif /* WITH_SDL */
diff --git a/ttd.c b/ttd.c
--- a/ttd.c
+++ b/ttd.c
@@ -52,7 +52,6 @@ extern void SetDifficultyLevel(int mode,
 extern void DoStartupNewPlayer(bool is_ai);
 extern void ShowOSErrorBox(const char *buf);
 
-void redsq_debug(int tile);
 bool LoadSavegame(const char *filename);
 
 extern void HalGameLoop(void);