At a Saturday night's (Oct. 25) concert in Buffalo, Kelly Clarkson brought out a cover that no one expected. The original 'American Idol' showed fans she's a true Swiftie with her own version of Taylor Swift's 'Shake It Off.'

"This is a church song, this is a soulful song," she told the excited crowd jokingly. "This is one of my favorite songs that's out right now. And you think you know it ... but you don't."

Clarkson poked a little fun at herself before beginning the song, giggling and admitting that she couldn't hear the correct key. Eventually, she laughed her way right into a soulful opening of the upbeat pop song, and it took fans a minute to realize what she was singing ... but as soon as they did, they erupted with applause.

When the players started playing and the haters started hating, Clarkson stepped it up a notch, bringing her rendition of Swift's song a little closer to the original. Clarkson added the riffs only she could pull off, impressing the crowd -- and Swift herself. When the 'Shake It Off' singer got word of the performance, she tweeted her excitement:

But really, is there any song out there Kelly Clarkson wouldn't 'slay'?

You Think You Know Taylor Swift?