As a blue player, one of my favorite cards of all time is Cheatyface [0] from Unhinged:
> Cheatyface {U}{U}{U}
> Creature — Efreet (2/2)
> If Cheatyface is in your hand, you may sneak Cheatyface onto the battlefield. If an opponent catches you right away, that player may exile Cheatyface.
Cheatyface is great because it's intentionally illustrative of bad game design. It's no fun to be constantly on guard for your opponent to cheat! MaRo actually went into this in a fantastic talk at GDC [0] a few years ago.
