It all went down at her show. She surprised fans with a Biebs cameo, and of course, fans went nuts. Not only did they sing his song, "As Long As You Love Me," but he sang her tune "Love Me Harder"...and he totally forgot the lyrics.
It wasn't just a little mess up, either. Everyone noticed, and footage made its way online ASAP. Like the pro he is, the Biebs admitted his flub. After the show he posted on Instagram, "Had fun with u Ariana, even tho I forgot all the words haha."
Watch Justin forget the lyrics while performing with Ari, below.