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X11 made a stupid mistake and defined 2 boolean types, BOOL and Bool, with different sizes.  My machine has 1-byte BOOL and 4-byte Bool.

src/video/x11/SDL_x11keyboard.c X11_InitKeyboard() declares BOOL xkb_repeat, allocating 1 byte on the stack.  It then passes &xkb_repeat to X11_XkbSetDetectableAutoRepeat(), which expects a Bool * pointing to 4 bytes.  When XkbSetDetectableAutoRepeat() writes its 0 or 1 to the Bool *, it writes 4 bytes to the 1-byte BOOL xkb_repeat.  This overflow corrupts 3 bytes on the stack.

If we are lucky, the 3 bytes might be unused padding for alignment.  I was unlucky.  I have an unusual macppc machine running OpenBSD-current.  The compiler is gcc 4.2.1 and enables -fstack-protector by default.  The compiler put BOOL xkb_repeat near the stack canary, so XkbSetDetectableAutoRepeat() smashed the canary.  When X11_InitKeyboard() returned, it found the dead canary, aborted the program, dumped core, and wrote "stack overflow in function X11_InitKeyboard" to /var/log/messages.

This bug caused every program using SDL2 on OpenBSD/macppc to crash when calling SDL_Init() to initialize the video.  Yes, every program using SDL2 on OpenBSD/macppc was broken, but macppc machines are old and rarely used, so I don't know if anyone other than me noticed the problem.

I made a local fix for OpenBSD's package of SDL 2.0.8:

Index: src/video/x11/SDL_x11keyboard.c
--- src/video/x11/SDL_x11keyboard.c.orig
+++ src/video/x11/SDL_x11keyboard.c
@@ -266,7 +266,9 @@ X11_InitKeyboard(_THIS)
     int best_distance;
     int best_index;
     int distance;
-    BOOL xkb_repeat = 0;
+    /* This must be Bool from <X11/Xlib.h>, not BOOL from <X11/Xmd.h>,
+       because the types have different sizes. */
+    Bool xkb_repeat = 0;
     
     X11_XAutoRepeatOn(data->display);
 
This is enough to prevent the crash and play Simutrans and SuperTux, but isn't a complete fix because there is also a wrong prototype of XkbSetDetectableAutoRepeat in SDL_x11sym.h.  I have not yet written and tested a complete fix.

<X11/Xlib.h> declares Bool as
#define Bool int

<X11/Xmd.h> declares BOOL as
typedef unsigned char  CARD8;
typedef CARD8           BOOL;

BOOL is an unsigned char of 1 byte.  Bool is an int, which has 4 bytes on most machines.  Writing through Bool * to a BOOL corrupts 3 bytes of memory.