1) Invasion portals can spawn inside walls, and that wall becomes un-diggable. Happened to me on floor 3 of necro bastion. Just infinite naga's popping out with no way to stop them!
2) Yeeks using their racial to take control of themselves seems to crash the game.
3) Can somehow dupe items via the itemvault. Didn't try to test it, but while just transferring some stuff I noticed that my guy somehow had made a copy of Black Robe. Server shenanigans no doubt.
4) On Classic UI, the map is messed up in a couple ways. First is that you can't right click on mini-map like you can in Minimalist to move vision over to that spot. Second is that if you pull the big map up with tab, it tends to cut off portions of big maps which makes it difficult to look over there considering the first way it's bugged.
5) Elemental Harmony + bonus masteries has some crazy/subtle/hard-to-pin-down bug where instead of them improving the effect, they sporadically just wipe it down to base instead. Had a summoner with a necklace of mastery + Tree of Life + some artifact mindstar that gave bonuses. With them all equipped the tool-tip said I should be getting 66% global speed boost from the fire, but it regularly procced for 44/54/56.
6) Invisible/Untargetable/insubstantial dudes that non-theless continue to cast stuff and melee you. This has been around forever, just these last few games I've had a Stone giant who kept sun infusioning, bastion downstairs room bosses (they seem most common) lighting storming, and an ooze in ruined dungion slime spitting. Have saved games/known locations of the stone giant and ooze if it'd help.
7) Sometimes, but not always, when you do the extended rest that waits for all your resources/cooldowns, if you have a bar that has no regen for it it will just try to rest indefinitely. Happened occasionally due to my mana bar due to no natural regen (seer escort).
Not 100% sure these next two are bugs or intended, but I found them counter-intuitive so I'll post them anyway.
8 ) Can't effect any of the summon-able things (spiders/totem/vampire/tentacle/etc) with Epidemic, though pretty much every other spell works on them.
9) Multiple spellstack on crit items bonuses get all lumped together such that if you have say, 3 of them, for a total when all stacked for 80, then if you unequip 2 of them and blast something else the bonus refreshes but it's overwritten with only that item's triple stack.
EDIT: Remembered another one, so added it.
10) Also not 100% sure if it's intended or not, but my reading of the tooltip would indicate it wasn't. Anyway, I'm 99% sure that corrupted negation sometimes also strips off my diseases and curses from people I hit it with.