Respawn mode + join-in-progress & spectators
On respawn-enabled maps, respawning does not occur for players who join during a round, and for players who switch to the spectator slots.
To reproduce:
1) start a match on a map such as Arkhangelsk or King of the Hill.
2) Join a team and spawn.
3) Switch to spectator.
4) Join a team again -- this time you will not spawn.