Albus Severus Potter, in the great tradition of the Mauraders and the deceased Fred Weasley, decides to prank his dad.
Except he miscalculates. He soaks a piece of instant skin with capciacin, cuts it into the shape of his dad's scar, and then manages to place it on his dad at some point.
It works. It causes Harry's scar to burn and itch, causing massive panic in the wizarding world when Harry lets it out that his scar is burning. THe whole wizarding world thinks voldemort's come back, and the whole thing spirals out of control before Albus can stop it.
Story ends with Albus facing the true wrath of Harry Potter.