One thing I've noticed on OSX (not sure if this exists on Windows or not) is that there's a problem where every once in a while, a monster will freeze in place and not animate. The monster is still active and attacks you, but you don't get to see it move. Eventually if you kill it it'll just skip to the dead monster.
I think this happens more with bosses than with regular monsters.
A short game of a couple of minutes might never run into this problem, but playing for an hour or 2 will.