FRIENDS fans have just noticed a huge blunder in Monica's wedding dress saga - which has gone unnoticed for 17 years.

In The One With The Cheap Wedding Dress, Monica fights with fellow bride-to-be Megan Bailey over a gown at a discount store in Brooklyn.

5 Monica bought the WRONG wedding dress in Friends and fans have only just noticed

5 Monica fought with Megan, pictured, a woman she met in an expensive bridal shop, over what's supposedly a cheap version of this dress

Monica is outraged because she met Megan earlier when trying on the 'same' dress in an expensive New York city store - and accuses her of trying to steal her gown.

With some help from Pheobe and Rachel, who wrestle the rival bride to the floor, Monica manages to bag a bargain and buy her dress at a discount price.

But fans have noticed that the dress the women fight over isn't the same dress at all.

The original gown had a halter neck, while the one the ladies fight over has a sweetheart neckline and lace straps.

5 But fans have spotted it's actually a completely different dress

5 Then when Monica later tries on her purchase at home, she's back in the ORIGINAL dress

And that's not all. When Monica later tries on the discount dress at home, she's back in the original one.

The error was spotted by one super fan on Reddit, who wrote: "Does this drive anyone else crazy?

"In the One with the cheap wedding dress, Monica tries on one dress at the expensive shop, then fights with that girl over a different dress completely - every time I see it, it drives me crazy!"

While another fan added: "When she shows Chandler the dress at the end of the episode, isn’t it the first one again too? (Right when they scurry back into the bedroom after she says she’s returning it)".

5 Monica ends up marrying Chandler is a completely different dress - after trading it for a favour from Megan

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Avid fans will know that Monica doesn't ultimately get hitched in either of these dresses - because she gives it to Megan in exchange for a booking with the Swing Kings band Chandler wanted at the wedding.

But this doesn't add up either.

Viewers have previously been told that Megan and Monica had different wedding days, so her cancelling wouldn't have helped Monica out.

Wedding planning, eh? It's full of drama...

In more Friends news, the director has recently revealed Rachel and Ross were never meant to go on a break… and it changes EVERYTHING.