One is the paladin promotion, if you enter the ruins and speak to the ghost you get the opportunity to check four notes scattered in the area, if you check all four of them before taking this quest then the quest will get stuck when you try to continue since the objective will be stuck at finding those notes.
(Which you can no longer interact with since you already checked them before.)

If you don't have a palading you'll just get stuck with four notes (And later on a disc.) in your inventory.
(Same with a amulet you can take later on if you don't have a hunter or what the orc hybrid class was called.)

For the dwarf shield guardian promotion there's also a dwarf in a cave you rescue and talking to him before accepting the quest apparently breaks the progress much in the same way as the above ruins.


Other minor bugs include the combat music not stopping if you've played for a prolonged period of time (Might be related to some memory leak?) and a quest from Jassad while still possible to complete you miss out on the relic dagger since there's a typo in the script file.
(Not a bug but a quest that you can't complete until after beating a game since the item needed is in the last dungeon and you can't return to the main world while doing this anyway if you explored and did what you could before the point of no return you'll get this relic and it will be stuck at level 1 since there's nothing left to kill or quest for.)


EDIT: Oh and just as a heads up there's three places where you need either a dwarf, a orc or a elf to access, they are mostly for the quest(s) for those classes but you will miss out on some enemy encounters and thus XP if you can't access these.
(Plus aside from the endgame dungeons these locations are pretty amazing looking.)