We're going to be as delicate as we can while we're talking about "American Pie", because it's one of the funniest films ever made, but there's some really gross moments in there and we shouldn't be talking about them in public. If you've seen the film, you remember this scene in Stifler's bedroom. You know what's in the cup, right? Good. Let's say no more about it.
Only we have to, because it's not the same cup anymore. When we first see it, it has a very clear blue rim. It gets put down, it gets picked back up, and now it's clear plastic. So, either there were two drinks in the room, and Stifler didn't actually drink what we all thought he drank, or somebody couldn't even be bothered to make sure they got a set of matching cups to shoot the movie with.