A Remembrall is a magical large marble-sized glass ball that contains smoke which turns red when its owner or user has forgotten something. It turns clear once whatever was forgotten is remembered.

Neville received one in his first year (which also helped Harry in becoming a seeker), but neither the books nor the movies tell us what he forgot. But if you look closely, Neville had forgotten his cloak.

11 signs you're totally Neville Longbottom from "Harry Potter ...
He isn’t wearing his school robes.

Do tell me how many of you knew that and had noticed it.