Still fighting mob after "Level Complete!" dialogue

Yesterday I joined one or two other people in a level. I saw a mob and started to attack it. The teammates were off on the other side of the level. In the middle of killing the mob, the "Level Complete!" dialogue came up.

I was going to post screen shots as proof, but I guess we can't upload attachments. Too lazy to post on some photo sharing site and link. The level was "Forest Haven - 1: Trouble in Forest Haven" and the mob is one of the zombie guys, and he's near a chest on the western side of the level.