Justin Bieber’s surprise cameos: Ultra, Ariana Grande concert

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As if downtown Miami didn’t have enough excitement this weekend with Ultra, Justin Bieber practically gave fans a heart attack when he joined Ariana Grande on stage in the middle of her hit Love Me Harder at AmericanAirlines Arena Saturday night. They grooved together, with Bieber dancing around and Grande kind of doing a sexy swaying bit in a shorter than short dress. Bieber 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.

Biebs did more gatecrashing on Sunday night, sending Ultra off with a bang. The 21-year-old joined Skrillex and Diplo jumping around on the Bayfront Park stage for the throbbing beat of Where Are Ü Now. Not sure we can call him a pop singer anymore. Or a kid. After his EDM debut, Bieber finally went to LIV as a legal adult, hanging out with Nicki Minaj, Meek Mill, Busta Rhymes and more.

More grown-up behavior: Bieber was also seen at P. Diddy's Ultra Music Festival bash at his Star Island manse that started in the wee hours Saturday and lasted until Sunday night. The marathon pool party featured flowing Ciroc vodka, Luc Belaire bubbly and tabloid dream come true, Khloe Kardashian and French Montana, who stayed at chez Diddy all weekend. Seriously, what gives between these two? The on-again, off-again couple continue to fuel reconciliation rumors as their lovefest continues all over town.

Earlier in the evening, they were spotted looking cozy at a Hennessy V.S. party at DREAM nightclub. The rapper was seen escorting the Keeping Up With the Kardashians star, in a black pantsuit with plunging neckline, to the back entrance of the SoBe venue while placing his hand on her backside. Joining their party was everpresent buddy Sean Combs, who posted a festive pic on Instagram, with his son Justin, newly 21.

Lesley Abravanel and Anthony Cave contributed to this report.