The theory goes that, after the events of Harry Potter, George used wizard time-travel technology (that is, the Time-Turner, a device that is used once and never mentioned again) to go back and start a new life under a new identity. And don't give us any guff about all the Time-Turners being destroyed, Harry Potter nerds -- we both know that at least one is floating around the black market, assuming J.K. Rowling hasn't already retconned a few back in.
Why It's Not That Crazy:
Before George's brother Fred died (uh, spoiler?), the twins opened a successful toy and candy shop called Weasleys' Wizarding Wheezes.
One of their most popular items was the Skiving Snackbox, candy that would give you a temporary nosebleed or vomiting fit so you could get out of class. In other words, the Weasleys had mastered magic that lets candy physically alter a human being. Like, say, this: