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.
Revealing mistake: As they're leaving together from Rosie's house at the end, Rosie stands up in the car to wave at people. The car sinks down with her weight when she puts her second foot inside the car, not when she takes it off the ground which is when all her weight would be on the car. (01:40:20)
Factual error: When Jack Black wants to be with Gywneth Paltrow, he decides to join the Peace Corps and leaves soon after. Anyone who has served in the Peace Corps or ever applied for service, knows that it takes months, and in some cases even years, to complete the application process. Therefore, it is impossible to decide to join the Peace Corps one day and commence service shortly afterwards. No one, even people who work at Peace Corps Headquarters, can swear-in as a Peace Corps Volunteer until after their first 3 months (out of a total of 27 months in country) of successful completion of training.
Continuity mistake: In the scene where Gwyneth is stripping at the end of the bed they start out with a front shot. She puts her hair behind her ears and there is none on the front of her. Then they switch to a back shot. She now has her hair pulled forward where it is lying on her chest.
Continuity mistake: In the scene in the restaurant when Hal is about to find out what Rosemary really looks like, Rosemary is sitting with a green napkin in her lap. When she starts to fall, from the booth breaking, the napkin is still in her lap. When Hal rushes over to help her up, the napkin is nowhere in sight.
Continuity mistake: When Hal's dad is on his deathbed, his hospital gown is wrinkled, including a large fold across his stomach. There is then a shot of Hal with his dad in the foreground and the gown is perfectly smooth. The next shot shows the dad again and the wrinkles are back. (00:03:20)