During the scene when Jim Carrey (Charlie) was caught with the lotion and photo of Renee (Irene), the camera pans the room when she is walking around, and you see a holey watermelon lying on the floor. It was from a cut scene. The cut scene is on the DVD issue.
I'm not quite sure where the slip-up comes into play. Are you saying that the watermelon was there, but then it cut to a different frame, and then the watermelon was gone? If this is the case... I believe it was an editing mistake, not a true slip-up.
It's hard to call this a slip-up. The watermelon is in the scene but the scenes that explain why it's there were deleted. Most likely they had to remove it at the last second for MPAA rating, and didn't have time to re-take the scene.
No, they deleted the sceen with the watermelon because of American Pie. While shooting the watermelon scene, American Pie came out, with a boy making love with an apple pie. They didn't want to copy them!