The suited German, sidelined with a knee injury, can be seen strolling towards the home bench in the wake of Steve Bould as the participating players ready themselves on the pitch behind them.
Noticing a young fan recording the moment on his phone and holding out his hand, Mertesacker stopped to shake it.
But the supporter pulls it away in the style of a classic gag.
This seemed to anger the Arsenal defender, who can be seen almost squaring up to the handshake prankster before perhaps thinking better of it.
A series of dirty looks were doled out instead.