People are still obsessed with Friends even now, fifteen years after the series ended. It's one of the most popular series people revisit on Netflix, too, and eagle-eyed fans have noticed a rather creepy element in the background of one of the show's earlier episodes.

In the Season 4 episode "The One with Rachel's New Dress," Rachel (Jennifer Aniston) and her date, Joshua (Tate Donovan) receive a surprise visit from Joshua's parents.

Rachel's attempt to have a sexy night in with her man goes awry when Joshua's parents arrive and she finds herself having to pass off her lingerie as part of a stylish designer outfit.

"See, I work in fashion and this is a real dress. 'They're wearing it in Milan… A part of my job is to wear the clothes and then I see how people respond and then I report back to my superiors at Bloomingdales," she says. "And obviously in this case, I'm going to report back ''USA, not ready.'"

Behind them, demon-like eyes flicker bright red before dimming again.

Wait, what? See for yourself:

Friends - Rachel & Joshua - Rachel's New Dress www.youtube.com

It seems to be little more than a goof caused by the show's lighting team, but it still left many fans intrigued.

Recently, Tate Donovan spoke about how difficult he found working with Aniston on the show to be.

"I was just happy to be on the team. The only bummer was Jennifer and I were breaking up at the time," he recalled. "And so that was tricky to sort of act, and act like we are just meeting each other, and falling in love, or whatever, interested in each other, when we're sort of breaking up."

Ouch.

This isn't the first time fans of the show have spotted random gaffes.

Here's a clip of Matt LeBlanc (Joey) mouthing David Schwimmer's (Ross) lines:

LeBlanc does it again in a different episode because why not?

This show will never grow old.