After some debugging here, cutting away code here and there, recovering from frustration, and some other magical moves, I found … a buffer-overflow of an itoa(). I love it. It did garble the stack, resulting in a restart loop. Pure magic, dark magic. I hoped for something honorable, like a compiler bug.
One of my favorite from http://natashenka.ca/posters/ seen at 30c3. There are more!