Ari's Bieber-rendition shouldn't come as that big of a surprise, though. YouTube user Jake Marra posted a mash-up of Ariana's "One Last Time" and the Biebs' latest single online. She later called it "genius" on Ryan Seacrest's radio show, and we're assuming that's where her inspiration to sing his song at her show came from.
We don't blame Ari for singing Justin's tune, anyway. We can't get that ish out of our heads. #ForReals
Watch a fan-shot clip of Ariana singing Bieber below. And check out the mashup if you haven't yet!