Blake Lively was secretly wearing sweats in this ‘Gossip Girl’ scene – 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.

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.

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!

Oct. 31, 201901:00