Blake Lively was secretly wearing sweats in this 'Gossip Girl' scene

Blake Lively's wardrobe fail in an old episode of "Gossip Girl" has gone viral, thanks to an eagle-eyed fan.
/ Source: TODAY

Serena van der Woodsen in sweatpants? You heard it here.

An eagle-eyed TikTok user spotted a major wardrobe fail in an old episode of "Gossip Girl," and now it's going viral.

In a scene from the sixth season, which features costars Blake Lively (Serena) and Penn Badgley (Dan), Lively is seen getting up from a restaurant table wearing a body-hugging coral dress. But seconds later, the camera pans over Lively as she makes a phone call — and suddenly she's wearing a comfy pair of sweats below the dress.

Blake Lively must have forgotten to ditch the sweatpants for this "Gossip Girl" scene.skyehanamaikai/ TikTok

The hilarious video quickly racked up more than a million views from fans who relish seeing wardrobe fails and other mistakes in major movies and TV shows.

Earlier this month, a TikTok user noticed that a pair of modern water bottles accidentally made cameos in a scene from the Oscar-winning film "Little Women,” which is set in the 1800s. Whoops! The bottles can be seen just behind Timothée Chalamet, who played Laurie in the film.

Eagle-eyed viewers noticed two contemporary water bottles behind Timothée Chalamet in the movie "Little Women," which is set in the 19th century.Amazon

Video of the flub spread like wildfire after a Twitter user pointed it out, too.

"PLEASE THE HYDROFLASK AND WATER BOTTLE I CANNOT," the person wrote. The tweet also included a close-up of the items in question.

And then, naturally, the rest of Twitter had a field day with it.

The snafu called to mind a similar situation last year when a Starbucks coffee cup made its acting debut in an episode of "Game of Thrones" ... only to be followed up by a stray plastic water bottle slipping into a scene in the show's finale.

We can't wait to see what sharp-eyed viewers will notice next!