I have experienced a VERY weird issue which has just happened to me.

I have used the "Search for opponent", it wasn't finding anyone so I Canceled the search. Then I went into the Challenges menu, and, suddenly, a multiplayer game started on background (while being in Challenges menu)!!! I was curious to see what would happen if I start a challenge, and, got this:
http://imageshack.us/a/img543/5458/challengem.png

Firing at the target has spent all the opponent's health to 0, and the game got stuck.

Uh oh, this doesn't sound good! I've informed the team about it. Thanks for the detailed report and image, this is a big help.