Why not? Because one of the plot points involves "The Drive," the John Elway-led march that resulted in the Broncos defeating the Cleveland Browns in an '80s AFC championship game. And in the movie (spoiler alert), it never happens.
As the title implies, a magical hot tub transports three old-but-grown-apart friends and a random nephew -- Adam (John Cusack), Nick (Craig Robinson), Lou (Rob Corddry) and Jacob (Clark Duke) -- back to a ski resort circa the '80s.
During their day in the past, Nick and Lou wind up at a bar, where the Denver-Cleveland game is on the tube. Because Lou remembers that the Drive ended in a touchdown with 37 seconds on the clock, he makes a bet with a moneyed sleazebag. If Lou's right, he gets to bang the guy's bimbo wife; if he's wrong, he'll orally pleasure Nick.
At first, the game progresses as Lou remembers it. But at the last instant, a squirrel distracts a Broncos receiver (probably Mark Jackson, who caught the TD) and he drops the ball.
Presumably, this happens due to the butterfly effect -- a small change in the past that alters what happens thereafter.
This twist is mostly an excuse for Lou to almost but not quite blow Nick -- and it doesn't make sense for several reasons. For one thing, the signage on view throughout the film references 1986, but the game actually took place on January 11, 1987. Moreover, the flick's conclusion revolves around (another spoiler alert) Lou staying in the past and using his knowledge of future events to become incredibly wealthy. But if the Drive didn't happen, why wouldn't other things differ enough to screw up his scheme, too?
Granted, the movie doesn't justify thinking too hard about any of this: Let's just say it ain't no Hangover. But the very idea that The Drive fell short, particularly in the wake of former Broncos coach Mike Shanahan obtaining the services of Donovan McNabb for his new team, the Redskins, while the Broncos are left with Brady Quinn, won't strike many Denverites as funny.
Call it Elway's revenge.