The thing about the How I Met Your Mother finale that pissed off so many fans is that it wasn't exactly a cop out—half of it was filmed in 2005, when the show began production, so it was the plan all along. Sure, it feels like you've been red-herring'd for a decade, but such is television.
Jason Segel, who played Marshall Eriksen on the show, had a pretty fantastic pitch that was turned down because it wasn't "romantic." Similarly, this idea is also a red herring, but it's absolutely fantastic regardless. We won't spoil it for you, as the element of surprise is the best part—just watch the clip above. Here's a hint though: It's more I Am Legend than [enter John Hughes movie here].
A couple of years ago, he shared the pitch with Jon Stewart on The Daily Show, and thanks to the Internet, you can watch that now.
It starts at the 4:25 minute mark.