Three Times Is Enemy Action

"Once is an accident. Twice is coincidence. Three times is enemy action," Ian Fleming. What if someone didn't take Albus' word of "harmless pranks" as Merlin-given gospel, but took a distraught boy's plea with the seriousness it deserves? AU to the Marauder's sixth year "prank," where everything's the mostly same, though Severus Snape's home-life is vastly different from canon.