Everytime I start writing a 3D rendering framework from scratch using OpenGL or Direct3D, I can guarantee that in the beginning I'll end up with a black, empty screen, wondering for hours why it is black and not showing the 3d geometry it's supposed to render. (Usually it's a wrong matrix, missing shader or simply the light turned off).
Same for Character Animation: Every time I start writing a character animation system, I end up with something like this:
Looks like some weight or joint matrix went wrong somewhere. I know I'll find the problem, but it's impressive that I'm always getting the same errors. :)