This occurs when you were unable to join a game server within three minutes of your queue popping. You do not receive a pug leave, minus karma or have your rank/stats negatively affected when your server slot expires before you were able to join your assigned game.
Support staff are unable to clear your server slot for you and your only option is to wait until the game you were meant to join has been completed or 45 minutes has passed, whichever occurs first.
In the very rare instance that your server slot says it has expired, when in fact, you have already completed your game, wait 10 minutes and press the "Leave Lobby" button. You do not receive any penalty for leaving the game lobby and may freely do so if it lets you.