The bug I always hated back when I played on the .my server was the bug where you would sometimes get stuck in a casting animation if you were hit while trying to cast a spell. This most frequently happens in venom dungeon because of poison. If you are casting and poison damages you, you will often get stuck in the casting animation and be unable to move or cast again until you used a potion, ate food, or got hit again. This made soloing as a caster such a pain. It would be awesome if this bug was fixed.
I think this bug might be somewhat related to the bug where the health you see on your client side becomes less than the amount of health the server says you have when you're constantly taking damage, but it's just a guess.