I've encountered a weird script failure in the last chapter. After throwing the last fireball into the volcano, I did not get a thunder and the first vent on the route to exit did not activate. I got stuck and the last checkpoint was corrupted too, because the fireball was gone. Interestingly enough, the bug reared its ugly head again after quitting to menu and replaying the chapter. I had to quit to desktop, restart the game and replay the chapter again. The bug was gone and I couldn't trigger it again. For the sake of completeness, it happened when I was playing continuously from the start.

I couldn't find this issue mentioned anywhere online, so perhaps I'll save someone in the future from a bit of bother.