It's been pretty well established that pop star Britney Spears isn't quite singing live during her Las Vegas Piece of Me residency at Planet Hollywood, but at least she's been dancing and miming to her own music, right? Not so much... at a recent performance, the sound mix for her Britney Jean single "Perfume" got all messed up, and Spears can be seen lip syncing to vocals performed by the track's songwriter Sia.

In an Instagram video taken by user britneysite and posted on Oh No They Didn't, Spears can be seen performing "Perfume" and acting like a live performer, but it's not the original version heard on Britney Jean... it's a remix done by Spears and Sia, and Sia's background vocals are far more audible than the billed star's.

As Billboard points out, there's a stunning acoustic mix of "Perfume" performed by Sia and Spears floating around on the web, and Sia has performed the song live herself... and it's totally obvious that Spears is syncing to Sia's version instead of her own.

Watch Spears' performance below, and compare it to the previously mentioned acoustic performance:

Seems like someone will be losing their job... and we doubt it's going to be Spears.

Despite lip synced performances, Spears' Vegas residency has been a critical and commercial success, and after an initial run, the show has been renewed through Feb. 2015. Spears will next tackle the Planet Hollywood stage tomorrow, Aug. 19.

Check out the full list of dates below:

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