Continuity mistake: When the Calvin Klein model throws his underpants at J.T, a couple shots later they are back on him.
Continuity mistake: At one point, at the beginning of the movie, the caveman gets mammoth poop all over his face, in the next shot it's gone.
Answer: It's a spoof of the Sarah Silverman song called "I'm F**king Matt Damon", plenty of versions to view on YouTube.
GalahadFairlight