Inspired by this YT-video.

A character is talking smack about someone else who promptly appears right behind them. Commonly Played For Laughs because the character is often oblivious, at least at first, about the person behind his back. A Flat What may be added at the end of the smack talk if he reacts to his friends's shocked expression when they see the other person behind him.

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Before I add any examples (from the video above, among others), I'd first like to know if this already exists.