Using @a and adding an exclamation point in front of a team name in the team= argument is broken if there's more then 1 person involved.

Just add two players to team Red, then call up @a[team=!Yellow] (doesn't matter if Yellow exists or not) and only one of the players will be selected for anything.

Screenshot shows an example. TrazLander & Gpmidi are on Red Team. Doing "say hi @a[team!=Yellow]" only says "hi TrazLander"