I wanted a command block argument that only outputs to players who where NOT on one particular team but also NOT on another so I applied:
@p[team=!TeamName1,team=!TeamName2]
but found that instead of working as I had intended it outputted successfully when I was on the first team listed in the argument and did not output when i was in the second.
I can confirm that I was playing in singleplayer with no other players online.

What I expected to happen was...:
For no output to occur when I was on either of the two set out teams whilst outputting successfully when I was not.

What actually happened was...:
An output occurred when I was on the first team listed in the argument and when I was on neither of the two whilst there being no output when I was on the second team listed.

Steps to Reproduce:
1. Create team Red using /scoreboard teams add Red
2. Create team Blue using /scoreboard teams add Blue
3. Create a command block with:
tell @p[team=!Red,team=!Blue] Hello
and place a button on the command block.
4.Test output when player is in no team by pressing button.
5. Join team Red using:
/scoreboard teams join Red
and press button to test output for when player is in Red team.
6. Join team Blue using:
/scoreboard teams join Blue
and press button to test output for when player is in Blue team.