Justin Bieber stunned the crowd at Ariana Grande's Miami gig on her Honeymoon Tour on Saturday night.

Bieber, 21, sent the fans into raptures when he appeared on stage for the surprise duet.  But, it could have fallen a little flat when he forgot his words!

The pair performed her hit Love Me Harder and Justin was supposed to come out and sing a verse - his voiced sounded great, but those in the audience singing along (if they could hear it over the screaming) must have been a little confused when he started singing alternative lyrics!

Luckily Ariana picked up on his mistake and took over and Justin just did a little dance.

Biebs apologized to Ariana saying;

“So I’m feeling a little bad. I forgot the words, Ariana, I’ve been trying to memorize them backstage for like an hour, I feel so bad I’m so sorry. Do you guys forgive me for forgetting the words? I feel terrible. ”

Justin then made up for it by singing his hits All That Matters and As Long As You Love Me with Ariana and they went off perfectly!

Check out the video below;