Deliberate mistake: During the finale, the backround looks like a sunset and there is a big door in the back. Watch closely and all of a sudden from one camera angle to the next (it's right after the big puff of smoke), the background changes to an elaborate background of layers of hearts, the door is gone, and there are about 60 more extras onstage. During the DVD Baz explains that they filmed the scene with both backgrounds and it just so happened that the beginning was better with one and the end was better with the other, so they put the puff of smoke in there to distract the audience so that they wouldn't see the change.
Deliberate mistake: During the "breaking up" scene with Jim and Michelle when they're inside the cabin, her hair is crimpy, but when she walks outside and gets into her car, her hair is straight. This is because Alyson Hannigan filmed this particular scene during a break in the filming of a "Buffy the Vampire Slayer" episode. (01:15:40)
Deliberate mistake: In one of the dogfight scenes a Japanese Zero crashes into a tower. It's a quick shot, but you can still see that what crashes is an American P-40 with Japanese markings.
Deliberate mistake: The 'NNN' news program shows a picture of some animal crackers in Frank's stomach. The problem is, they are still all in one piece and are pretty big. I don't think Frank would swallow them whole.
Deliberate mistake: When Mark is catching a cab outside "JFK" airport, the cars are driving on the left hand side of the road - if they were in America they would be going the wrong way (the director points out in the commentary that this shot was filmed in London to save money).
Deliberate mistake: Throughout the movie, you can see the size of true-size Rosemary, particularly her waist and legs. Notice the size of undies she was going to purchase when she and Hal first meet. Notice the size of her bathing suit bottom. However, in the scene where she throws her purple nightie undies at Hal and he catches them: the panties are WAY TOO BIG. Especially the leg holes, which could go around her true-size waist.