In a clip from the performance, shared on Instagram by user britneysite, the 32-year-old star can be heard singing to Sia's version of the song.
Spears is said to lip sync her way through her 90-minute Vegas gig, but this is the first time the singer has used a track featuring a different performer. While Spears' vocals can allegedly be heard in the clip, Sia's voice stands out. Billboard reports the singer likely uses Sia's track "as a in-monitor guide" when she sings, but the track must have accidentally made its way into the live show.
Sia is credited as one of the writers of Spear's "Perfume." You can hear both performers singing a studio version of the song below.
Spears made headlines in July after a non-Auto-Tuned version of her track "Alien" leaked online and found her singing out of tune. Track producer William Orbit released a statement at the the time claiming the leaked song was a just a "warm up." Both "Perfume" and "Alien" are featured in the singer's eighth and most recent studio album, Britney Jean.