Shutter Island's famous plot twist was given away by a prop Leonardo DiCaprio used in his scenes

Shutter Island is one of the most mind-bending films ever made, having been released in 2010.

People (me included) are still getting over the film's ending, with it ranking highly among the best movie twists ever.

But as we've had a good 14 years to analyse the Martin Scorsese masterpiece, it was discovered that there was a prop that Leonardo DiCaprio used in scenes that hinted at the film's shocking finale the whole time.

I think it goes without saying here that there are spoilers ahead.

The psychological thriller was adapted from the bestselling book of the same name, written by author Dennis Lehane.

It follows Teddy Daniels (DiCaprio), a US Marshal who is sent to an institution for individuals deemed criminally insane to investigate a missing patient.

Viewers of the film will see flashbacks that reveal Teddy's complex history, but it is not made clear what happened to him.

Then the big whopper: at the end of the film, it is then revealed the US Marshal is actually a patient at the institution himself, as he was sent there for killing his wife after she drowns their children.

The shocking ending to the film was right in front of us the whole time. (Paramount Pictures)

It turns out that the whole 'missing patient' narrative was something Teddy came up with to help him deal with the trauma.

The staff decide to go along with it to see if it will help him accept reality, but it doesn't work and he's given a lobotomy instead.

It's a corker of an ending - which is actually very different in the book.

After finding out who Teddy really is, and re-watching the film, viewers have noticed one tiny clue which points to the truth all along.

A video uploaded by movie buff @imeugeneclark explains that a prop used in the film reveals that Teddy was a patient all along.

"Did you know that in Shutter Island a small detail reveals Teddy's true identity early on?" they explain.

"Of course, a big plot twist of the movie is finding out that Teddy Daniels isn't in fact a US Marshal but actually Andrew Laeddis, a patient at the mental sanatorium he is investigating.

Teddy is never allowed to light a cigarette by himself. (Paramount Pictures)

"This twist is actually given away as every time that Teddy goes to smoke a cigarette, someone else lights it for him.

"This is because patients on Shutter Island are not permitted to carry lighters around with them."

Other fans pointed out more clues, including the fact that Teddy's formal clothes are too big for him (because they aren't his) and that his 'partner' Chuck struggles to take off his holster before entering the facility, because he isn't used to it.

"It's one of the earliest clues that his partner isn't who he says he is, since someone with his supposed background shouldn't be struggling with such a simple action," they wrote on Reddit.

Others claimed the reason why Teddy doesn't light cigarettes is because of his past.

"Its actually because he’s terrified of fire because of his past, same reason he’s scared of the water throughout the movie," they wrote.

Meanwhile others are still convinced that Teddy was a US Marshal and he was simply tricked.

"To this day, I still believe that they tricked him and that he’s in fact a federal marshal," they commented.

What do you think?