Justin Bieber loves making surprise cameos at music festivals!
In March, the pop star crashed Skrillex's set at Ultra Musica Festival to perform as an energetic robot!
On Sunday, the singer surprised audiences when he came out on stage during Jack U's set at the Hard Summer Music Festival in Pomona, California!
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The 21-year-old rocked a grungy Metallica T-shirt as he hit the stage with Diplo and Skrillex --when the four artists...