Disclaimer: J.K. Rowling owns Harry Potter, not me.
Fred and George Weasley had many unspoken rules between each other, among them: never test potentially rash inducing products before a Quidditch match (ever again), never get a detention from Professor Snape, and always wear the correct Weasley sweater on Christmas. Nobody believed they would voluntarily wear the right letter anyway. The twins cackled as their family looked at the letters on their chests and called them the opposite name.
Percy scowled at George with his arms tied up in his own sweater, 'P' for perfect prefect Percy. "I do not have time for any of your pranks Fred. I was working on an important essay. Go bother Ronald."
"Christmas is a time for family, Perce!" George exclaimed, but Percy merely wrenched himself from the twins' grip and walked away fuming. "The day Percy smiles and jokes is the day the world ends."
"I agree, Gred. You know -" Fred began.
"Hey! I have looked all over the bloody castle for you two!" The twins' best friend Lee Jordan yelled from down the corridor. He joined Fred and George on their way outside. Lee glanced at them and claimed rather smugly, "I love Christmas. It's the only day of the year I am sure I can tell you apart."
Outside, the trio decided a snow ball fight was in order. "I want George on my team," Lee said grabbing the twin with the letter 'F' on his chest. "You guys always pair up." Instead of replying, Fred spelled a pile of snow to land on Lee. The twins took off with a laugh as their crazy friend chased them through the grounds starting an epic snow ball fight which left no one unscathed. Even an innocent Professor Quirrell, who happened to walk by at the wrong moment, gained a few snow balls to the head. It seemed Fred and George would always be switched, even wearing the right letter.