It's another anticheat, which banned me because I was using a SKIN ingame, which let me see through some things. VAC wouldn't ban me because I didn't actually cheat in the game when it allowed me to.
-BTW this skin would be downloadable on fps banana, it is NOT a hack or cheat.
There are a few servers that use KAC on CS:S and TF2.
I thought it'd be necessary to add that it is KAC banned. =\