When your match is at the end of the game usally u get the score counter in your screen with all you leveling up and such. Sometimes your screen just gets a gray overlay but u can still play and kill all standing still enemys. This happend twice to me. Once where i was the only one who got in the glitch and the second time i was with someone else.
It seems to happen when you exactly click to shoot when the timer is at 0:00.