Lemme take you back to the year 2000 or the year 2005, depending on whether you're more of a ~book~ or ~movie~ person.
As we all know, at the end of Harry Potter and the Goblet of Fire, Harry is transported into a graveyard via Portkey, where he watches Voldemort return to his full strength.
But Tumblr user Miraniel noticed something interesting: Namely, the Portkey (aka the Triwizard Cup) acts kinda strangely.
Here's a summary of Miraniel's theory: Portkeys usually only go one way, but the cup goes two ways. Not only that, but it returns to the front of the maze, not the center where it originally was.
The theory is that a Death Eater — or even Voldemort himself — was going to travel back to Hogwarts using the Portkey after Harry was dead, and join Barty Crouch Jr. as an undercover agent.
How, you ask? Well, with Harry dead, Voldemort has a full head of hair that he can use to make plenty of Polyjuice Potion, so he could return disguised as Harry, the triumphant victor of the Triwizard Tournament.
Voldemort would just have to pose as Harry long enough to get close to Dumbledore, when he could easily assassinate him and eliminate his one true remaining threat...and accidentally become master of the Elder Wand in the process.
But — thank goodness — none of that happened, thanks in large part to Harry's thick plot armor and his love of Expelliarmus.
Miraniel goes into much more detail in the full theory, which you can read here.