Neville Longbottom is best known as the timid Gryffindor who initially struggled to understand why he was sorted into the bravest house at Hogwarts.
He was also very absent-minded and needed to be reminded of things often by his Gran, Augusta Longbottom.
Remember in the first Harry Potter film, when he receives a Remembrall during mail delivery?
Well, as it was explained by Hermione Granger, the school's cleverest witch, if a Remembrall turns red, that means you've forgotten something.
And of course, Neville's filled with red smoke, which begs the question: What did he forget?
The only explanation that ~sort of~ holds up is that Neville probably forgot to wear his robes.
During that scene, everyone else is wearing their proper attire.
Everyone except Mr. Longbottom.
Oh. My. Gawd.
But wait! There's a catch. *queue disappointing music*
The only issue with this theory is that right before the the first years are sorted, Neville is spotted wearing robes.
See:
And if it wasn't his robes that he'd forgotten, maybe his Gran was just sending him a reminder to bathe regularly, as most grandparents would do.
Perhaps we'll never know what he forgot.
H/T Movie Pilot