'Hidden' Harry Potter theme in each movie's opening credits spotted by fans

qwThough the final of the eight Harry Potter movies landed in cinemas a staggering 13 years ago (I know, anyone else feel old?), this hasn't stopped fans of the series keeping their eyes peeled for unnoticed 'easter eggs'.

And whilst watching the star-studded blockbusters back recently, one batch of Wizarding World fanatics picked up on a 'hidden theme' appearing in each of the film's opening credits - claiming they failed to notice it before.

As we say, there's nothing Harry Potter fans love more than reviewing the films in the hope of spotting something they'd ever seen before.

Just last month, some viewers were blindsided by a deleted scene which seemed to imply that Draco Malfoy redeemed himself in the grand finale against the Dark Forces.

And just weeks earlier, fans admitted they didn't realise that a much-loved This Morning star was also filmed in Fred and George's Weasleys' Wizard Wheezes in Diagon Alley.

It also turns out that if you're looking hard enough, you'll come face to face with a 'hidden theme' right at the beginning of each movie.

In fact, you'll see that the Warner Bros. logo in the opening scene actually darkens with each film.

The Warner Bros. logo gets darker and darker with each movie (Kidspot/Warner Bros.)

They've also speculated that as the franchise's antagonist Voldemort grows stronger as the series continues - following his regeneration in the fourth movie, Harry Potter and the Goblet of Fire - the Wizarding World itself became bleaker.

And they believe that the change in colour scheme during the initial scene of each film symbolises this fear.

A snap of the ever-darkening movie introductions was shared on the Australian parenting and lifestyle brand Kidspot's Facebook page.

While some - potentially newer - fans of the franchise may not have known about the mysterious credits, others couldn't help but boast that they'd noticed the hidden theme the whole time.

"Ah yeah cause each film gets darker in theme, was that not obvious to people?" one jibed.

Another added: "This was pretty common knowledge for any movie buff."

The Harry Potter movies have become a Christmas staple (Warner Bros.)

The revelation comes just weeks after another batch of fans watching Harry Potter and the Philosopher's Stone clocked onto something rather baffling in a Neville Longbottom scene.

If you cast your mind all the way back to when Harry, Ron, Hermione and their pals were in their first year at Hogwarts, Neville was gifted an magical item known as a Remembrall.

This described a sinister little orb that contained smoke, which turned a deep shade of crimson when its owner misplaces or loses track of something important.

"I've read about those," Hermoine exclaims in the iconic scene. "When the smoke turns red, it means you've forgotten something."

Neville then replies with one of his character's most famous lines: "The only problem is, I can't remember what I've forgotten!"

Has the mystery of Neville's forgetfulness finally been solved? (Warner Bros.)

And apparently, it's taken until 23 years since the movie landed on our screens for some viewers to clock what it was that the Gryffindor favourite had misplaced - his robes.

In the scene, whilst everyone around him could be seen wearing a lengthy black cloak over their uniform, Neville is without his.

"I was today years old when," one commented on social media today.

Another confessed: "I thought the whole time it was because he couldn’t remember what he forgotten."

"Okay... It took just 20 years", another added before a fourth continued: "I never noticed when I was younger."