When changing a player's team, a command block will return true when it shouldn't.

Steps to reproduce:
1. Create teams "testTeam1" and "testTeam2"
2. Place a command block with a comparator
3. Insert the command scoreboard teams join testTeam1 @p[team=testTeam2]
4. Activate the command block.

What I expected to happen:
The command block would return true IF AND ONLY IF
1) the team exists
2) the player could be found and
3) was added to it.

What happened:
The comparator turned on, even if the player isn't on the correct team (or any team). Note that although the comparator turns on, there is no text in the bottom box explaining that a player has been added to that team, because the command block didn't run the command successfully, so the comparator shouldn't turn on.

----------------------

This might be part of a larger set of improperly returning comparator values. I noticed this issue began in the last few snapshots where I had not had the problem in early 1.8 snapshots. Please see MC-15567 for some other examples. I wanted to post this since that bug hasn't been updated in a while and the team joining issue only became a problem for me a few weeks ago.