While that might not seem like a big deal to some of us, it was to some big time Beliebers. So much so, that Justin had to explain things on Instagram.
Justin posted that he was "singing to the playback," and that means "the actual song is playing 'while is(sic) sing' so I can dance." Eh, we get it. It IS hard to dance AND sing at the same time. C'mon, we all know Justin can really sing.
He's not the only artist to use back-tracking, either. Beyonce, Madonna and even Cher is known to use playback during live shows.
Think it was necessary for him to explain himself? Honestly, we'd rather know what was up with those sexy moves with Ariana...