On that fateful night back in 2004, Ashlee performed her first song, "Pieces of Me," without a hitch. But, when it came time for her second song, vocals for "Pieces of Me" began playing, again – and Ashlee… kind of panicked and broke out into an awkward hoedown.
Once she realized there was no coming back after her little jig, she held her head down and sauntered off stage, leaving her band to continue playing for around 30 seconds before SNL cut to commercial.
Simpson re-joined host Jude Law at the end of the episode with a brief explanation.
"I feel so bad my band started playing the wrong song and I had no excuse so I had to do a hoedown," Simpson said. "I'm sorry!"
ET caught up with Simpson two nights later at the Radio Music Awards, where she fessed up to singing over a track and spilled details on the incident.
"I think my drummer got like really excited and maybe a little bit nervous and he pressed the wrong button," Simpson explained. "I was like okay, I think I've done the hoedown long enough -- maybe I should walk off."
While blaming her drummer for the song mix-up, she blamed her decision to sing over a backing track on her severe acid reflux.
"If you ever go out to one of my shows or anything like that I never ever -- I'm always singing my heart out," said Simpson. "I have severe acid reflux and it was just acting up that day."
While many other artists are known to sing over tracks for live performances, Ashlee insisted this was her first time doing it.
"So many artists do that," Ashlee said. "Of course the first time I do it… I’ll never do it again!"
She was able to shrug off the incident pretty quickly – and she was actually invited back to SNL a year later where she performed new songs without a hitch.
Ashlee's been busy in the decade since the flub.
In 2006, she ditched her dark roots and went back to blonde – but that wasn’t the only thing different about her appearance. She also rocked a brand new nose.
In 2008, she got hitched to Fall Out Boy bassist and songwriter Pete Wentz after two years of dating -- and took on his surname, making her legal name Ashlee Simpson-Wentz.
Only two weeks into their marriage, Simpson announced she was pregnant with their first child, Bronx Mowgli Wentz.
In 2011, Simpson filed for divorce from Wentz citing "irreconcilable differences."
Last year, she began dating Evan Ross, son of music legend Diana Ross, and the two married in August 2014 at Diana Ross' estate in Connecticut.
Take part in Throwback Thursday and check out this flashback stage flub.