When using the tag function there is a bug. To describe it we must set up an environment: We will use "T" for our tag's name, "O" for a scoreboard objective, and "A" for another objective.
So we want to use a tag and a certain scoreboard score to do something in a map, and we put in "/execute @a[tag=T,score_O=2] ~ ~ ~ scoreboard players set @p[score_O=2,tag=T] A 0"
Simple, we should see a player with T tag and O at 2 have A set to 0. Of course, if that happened I would not be here.

What happens is regardless of the score for O there is an output because I have the tag T implemented on the player.
So basically, regardless of anything but the tag's presence the command is successful.
Now, it is possible I have messed up my command, but, it appears to be a good command that should work in theory. Therefore I have come to say this is a bug that needs to be fixed or at least looked into to see if it is one.

Thanks for your time.