Justin Bieber -- Blankety Blank Blank Blank ... I Forgot Lyrics to Ariana's Song! (VIDEO)

Justin Bieber lost it with Ariana Grande Saturday night in Miami.

Biebs jumped on stage for a surprise performance with Ariana. They sang her hit, "Love Me Harder' together, and he sounded fantastic ... until he just spaced out on the words.

Even though it was a bonus for the crowd, Justin profusely apologized, saying he studied the lyrics before coming on stage.