I'd start with the Warlock Frigid Darkness Encounter power. Quoting the "hit" section:
Hit: 2d8 + Constitution modifier cold damage, and the target
grants combat advantage to all of your enemies until
the end of your next turn.
So I'm envisioning the following scene:
Bob the Warlock: So I hit the BBEG with Frigid Darkness for 15 damage.
Ted the Rogue: Sweet! Now he's granting me combat advantage!
Bob: I'm sorry you feel that way.
Ted: Why's that?
Bob: Because Frigid Darkness only makes the guy grant combat advantage to my -enemies-. That means I gotta kill you now.
Ted: WTF?!
Since any number of heads is better than one, I imagine we can spot those a lot quicker.