As if downtown Miami didn’t have enough excitement this weekend with Ultra, Justin Bieber practically gave fans a heart attack when he joined fellow pop star Ariana Grande on stage for “Love Me Harder” at AmericanAirlines Arena Saturday night. They grooved together, with Bieber dancing around and Grande kind of doing a sexy swaying bit. Bieber, seemingly on a self image reboot, admitted to the Boca Raton cutie that despite attempting to memorize the lyrics backstage he flubbed a few lines. Grande waved it off because it was doubtful anyone could hear him with all the screaming. Bieber stayed for a few more songs, much to the fans’ delight. Biebs did more gatecrashing Sunday night, sending Ultra off with a bang. The newly legal singer joined Skrillex and Diplo for a jump-around on the Bayfront Park stage for the throbbing beat of Where Are U Now.