Probably the oldest bug in TF2, if enough people join a team at the same time, you may find yourself trapped in spectate. But you're actually in an unassigned team, which breaks the use of the changeteam (your period key) command.
Forcing you to have to use the jointeam X command, spectate, or open the motd (Doesn't always work). Our fix will attempt to reopen the team join menu in this event, allowing you to get on a team.