I log into the creative test world I made in 1.2.5, and for the first time ever, it crashed the game. It would start loading the chunks, but almost immediately it crashed.
I thought I would join my singleplayer test world like normal.

Several times, whenever I tried to log into my world, Minecraft crashed.

I don't know if this is easily 'reproducible', but I'll attach one of the crash reports if anyone knows how to read them...