I've encountered a couple of rather nasty bugs in MotB. I thought it might be a good idea to have a thread specifically for them, spoiler-free if possible, so people would know what (not) to do and when to save.
Here's the ones I know of so far:
* Ice Troll Lodge initiation quests. The Test of Endurance and the Test of Combat sometimes leave you in limbo if you fail them in certain ways.
* Facing Okku. Sometimes if you die but your companions are alive, you're again left in limbo: you can continue playing but you won't become a Spirit Eater and the plot won't progress. (Credit: Maylander)
* Immortality. Happened to me twice on the lower level of the VotDG in SM: PC's hit points won't drop below 1 under any circumstances. Appears to clear up when leaving the level.