A few co-op bugs I have noticed when playing:

1) I used hammer based skills. Anvil's Woe and Flight of Gaavanir. Both of these randomly stop working in co-op throughout gameplay. Sometimes they are restored after loading new area or some other event.

2) The other player observes my character teleporting around rather than moving smoothly. They appear smooth on my screen. This happens randomly and we usually start with my character moving smoothly and after some time there is teleporting. They also have zombie summons and sometimes I observe their summons teleporting around rather than animating smoothly. Changing party leader doesn't change who is affected.