1. The first time Fred and George blew something up it was the only toilet in The Burrow. Bill had to shower for his first day of school by magic in the back garden. The gnomes laugh over it to this day. The twins were three.
2. George is a world-class secret-keeper. Unlike Fred, George was even capable of keeping secrets from his twin, though he'd always tell him eventually. There was one secret, however, that George never got to reveal to his brother and it bothers him daily. That secret was how he felt about Weasleys' Wizard Wheezes' employee,
Verity Thruston, to whom he eventually opened his heart.
3. Fred once put Impossible-Tangles Gum in George's hair. George punched him in the face for using him as an unwilling test dummy and Fred immediately removed the gum with a diffindo charm, regrowing George's hair with the antidote. The prototype gum, which was supposed to fly from the consumer's mouth into an unsuspecting siblings' hair (and thus remain un-removable, causing tremendous tangles until rendering the victim practically bald) was quickly cut from their inventory. It was their only true fight and the point at which they agreed to never test products on one another again. Percy then quickly became their favorite test subject.
4. Since Fred's death, George has had trouble sleeping. It is strange and disturbing for him to not hear the reassuring breathing of his twin next to him.
5. Though he'd never admit it, when they were fifteen, an easy prank went awry and George's hair turned gray. He hides it with a special Weasley-invented dye because a simple color change charm is ineffective against the discolored streaks. Fred took George's secret to his grave.
6. When the twins were young, Fred came down with Dragon Pox and had to spend the night at St. Mungo’s. Split for the first time, George had spent the night curled up in Fred’s bed, upset and crying.