There have been problems in the current version in filling the slots correctly. Seems to come up at 10 players. It seems the last slot can't be joined by new players and players sometimes can't switch to the last open spot. Removing such an open slot almost invariably puts a bunch of random people as spectators.
It's mostly just weird behaviour with the 10th player joining. Recommend you just test out 10 players in a lobby and see what happens. Here is a video (Download not 'watch' for acceptable quality) from a game 3 hours ago where the 10th player joined and people got bumped into spectator slots that weren't there prior to joining.
Happens on the new build and I had it happen in a 3v3 also. Starting symptom is that clicking on an empty slot doesn't move the player to that slot. Also sometimes the lobby contains players equal to the number of slots but an empty slot is displayed and you can see a player without a slot typing into the chat (this takes often the form of a new player joining "over" an existing player). When it occurs it can be made go away by removing and adding slots or allowing more spectactors. It also moves people into spectators while the game doesn't allow any.
I fixed one bug related to this problem... but I apparently did not fix it (I am unable to repro the problem). If a player is added 'over' another player it would create an un-joinable empty slot. I'll look at this again.
