Desync issues.
When you're playing PvP, it sometimes loses the connection between the clients. If you kill their creatures, then do whatever else, then end your turn, sometimes they don't see that you've killed their creatures. When they end their turn, and send what game data they have to your computer, their creatures aren't dead, so reappear. It's been known about for a while.
That said, because it's been known for a while, there's lots of posts on this in the bug section. (down the bottom of the main page.)
