Quote from OMG its a duck:
So I think I just found a really shitty glitch in the lava ball fight: He rotates according to your FPS, so if your FPS is less than 60 (like my laptop is when I'm recording), he won't spin the same way when he rolls at you the first time, so you can't quick kill him by shooting him before he hits you. This really stinks.
I think i have find the same kind of glitch against mushroom, i wasn't able to quik kill him, and that was the first time that happen.