In 14w08a i just discovered a bug whereby Minecraft crashes when i opened a commandblock.
I'm not sure how to recreate this bug or if it actually is linked to commandblocks, but this is what happened:
I was playing around with commandblocks set to a command like
i selected a region bigger than 4096 blocks total so i split up the clone command into two commandblocks.
after i did that the some parts of commandblocks began to shift and various other commandblocks got triggered, so i can't really tell whether it is the /clone command, the /tp command, the /testfor with a target with rx, rxm, ry and rym, or if it even is a command inflicting it.
after this i just tried to open a commandblock to fix it and the game just crashed.
I've put a crash report in the attachement if you would need it.
To me the crash report looks really short actually and its nothing I really understand.
I don't think this has too high of a priority because to me the crash sounds kind of random and after I tried to recreate it nothing really happened.