The Strokes frontman Julian Casablancas surprised audience members Tuesday night when he snuck onstage in a Win Butler mask during an Arcade Fire show.

Arcade Fire has taken to wearing large paper mache bobblehead-style masks in recent months, and their show last night at Lollapolooza Argentina was no exception. The band had started to sing their song "Normal Person" before the real Win Butler ran on stage and unmasked the imposter.

"What the [expletive] is this?!" the Arcade Fire frontman asked Casablancas in mock rage. "What the hell? What is this? Not cool!"

Casablancas blew kisses to the crowd as Butler took over the microphone and ushered him off the stage. The real Arcade Fire singer then joined his band as they resumed their performance of "Normal Person."

The Strokes singer's prank isn't the only trouble the masks have caused for Arcade Fire. Someone recently made off with band member Richard Reed Perry's mask at their March 18 show in Bridgeport, CT.