Britney Spears failed to lip sync last night in Las Vegas.

According to E! News, a concertgoer taped the pop star missing her cue as she performed "Alien" to a sold out crowd.

The star also had a special guest during the performance of her hit song "Freak Show."

Nicole Richie was invited on stage to crawl around on her hands and knees with a leash around her neck.

Britney, of course, held the leash as she performed.

Richie has been open about her epic love for Britney. According to Metro, the "Simple Life" star had previously said, " I have been a huge Britney fan since I was 16 and have been to all of her concerts. I think I went to five shows of her last tour."

Richie posted a picture on Instagram of her and the 32-year-old mother of two hugging on stage with the caption "#GOODBYE."

Spears tweeted later on, "Can we all give a round of applause to @NicoleRichie for playing along in the show tonight? U looked hot crawling on all fours."

She then tweeted to Richie directly saying, "Thanks for dancing tonight!"

We're sure that made Richie's week.

The Britney performing in Vegas is different from the one described by ex-boyfriend Adnan Ghalib, who said that he's "happy to see Spears performing in Las Vegas and getting her life back in order."

The New York Daily News reported that Ghalib recalled his relationship with Spears by saying, "She was deeply depressed and distressed and the most unhappiest person I had ever seen in my life. This wasn't drugs, but like someone suffering post-traumatic stress."

According to New York Daily News, most of her anxiety came from her battle with ex Kevin Federline when trying to regain custody of her sons, Sean and Jayden.

Despite her hurt and desire to leave the spotlight, it seems that Spears has bounced back from her performances.

Ghalib states, "I have seen firsthand that she is a music genius and very intelligent."

He continued, "She will always be Britney Spears - music superstar - to the world because of what she has achieved."