Hey, Upper East Siders.

It looks like Serena van der Woodsen had a small styling slip-up during season six, episode four of Gossip Girl.

The costume confusion was first pointed out by

Hey, Upper East Siders.

It looks like Serena van der Woodsen had a small styling slip-up during season six, episode four of Gossip Girl.

The costume confusion was first pointed out by TikTok user @skyehanamaikai. In the episode, titled "Portrait of a Lady Alexander," Blake Lively's character is seen dining at a restaurant with Dan Humphrey (played by Penn Badgley). At first, she is seen wearing a coral-colored dress with bare legs. But as the scene continues, the S star is spotted with a pair of dark sweatpants underneath her ensemble.

The clip from the 2012 episode immediately went viral and garnered more than one million likes.

"I am a huge Gossip Girl fan and have probably seen the show more than 15 times," one commenter wrote. "I can't believe I missed this."

"I still stan Serena even with her sweats," added another.

While the reason behind the added layer wasn't confirmed, some viewers posted theories on TikTok.

"LOL, I worked on set, it's because she ripped her dress," a follower wrote.

"From my experience, if it's cold, most actors put layers on between takes and sometimes leave them on if the shot is from the waist up," guessed another.