Summary:
Players still in lobbies after leaving them.
http://i.imgur.com/aHTZA.jpg - in this lobby, Joe Vega was the host but he's not actually there, despite the game lobby thinking he is. Now no one can become the new host and the lobby stays there until it's fixed.

Repro:
Not sure how to actually do it

but to see it happening -
1) Go to private games
2) Join any lobby
3) Look in "Participants" and see if any players in the game are missing.
4) Yep, they're missing.

Result:
An AFK ghost player is the host of a lobby that takes up space.

Expected result:
That player left.