Ariana Grande performed in Miami last night, and a few minutes into her hit "Love Me Harder," the Biebs joined her onstage. He's certainly used to belting it out in front of thousands of screaming fans, so we were shocked to see that he totally blanked on the lyrics in the middle of the song!

In his defense, it's not his song, and afterwards he apologized, saying that he studied the lyrics before the show. Bieber also tweeted, "Thank you @ArianaGrande and thank you Miami. I missed the stage. See u soon ;)"

Still, Ariana sounded fantastic and Bieber's "I forgot the words" dance moves were pretty awesome ... check it out in the video above!