Happened today.. after a kill, when trying to lose the iron sights, it got sort of stuck, and was just floating there. I swapped weapons to see if it goes aways.. did not. Swapping gun back and forth fixed it eventually.
http://steamcommunity.com/sharedfiles/filedetails/?id=152922368
http://steamcommunity.com/sharedfiles/filedetails/?id=152922368