I thought the nastiest bug... at least I keep seeing... is:
You get locked on to by monsters...
Manage to get away through a portal...
Go dump your loot/gold...
Come back...
Being visible with pets our mounted... or 100.0% hidden with 100.0% stealthing (or more)… and the monsters that locked on to you are still locked on like Gorilla Glue.
If you are hidden, you have to be extremely careful you dont trip over one of them... but nothing you can really do, considering you have to be FREE of them to cast and try to kill them. And some of the newer monsters tend to SPEED UP when you cast spells on them. Like they get hyper and crazy on you.
Hyper and crazy also works when you're working a tamer and want to get something off your pet, thats bugging your hunt. THEN the monster switches, disturbing the whole flow. BUT... it hardly works out for the monster when its being eaten to death for its bugging the master.