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Team Fortress 2 is a game about strategy, tactics,… and cheap laughs.
A video has cropped up on youtube recently depicting a rather hilarious scene involving a Red Spy and a Blue Medic. The Spy uses his disguise kit to assume the appearance of member of the blue team while the Medic unknowingly uses his Medigun to heal him (a common folly for Medics in game). The Spy than sprays an image on a near by wall that reads "FYI I am a Spy" and when the Medic turns to look at it, he gets backstabbed.
Since then, variations of the "FYI I am a Spy" spray have appeared. Valve, the creators of TF2, have also added a special spy achievement in their Spy Achievements Pack earned for backstabbing a Medic that has healed you in the last 5 seconds (There is also a variant of this achievement for Medics that involves using your bonesaw to kill 5 enemy spies who have been calling for a Medic).