Continuity mistake: When Prince Henry first approaches Leonardo da Vinci and then chases after the thief, the horse he rides on is dark bay in color and has no markings on its face. After he shouts, "Ugly peasant bastard!" in Henry's next close-up he rides a black horse with a white 'L' shape mark between its eyes. (00:20:20)
Factual error: After crashing on the asteroid, A.J. walks outside surveying the wreckage while debris is strewn out burning on the ground. This is wrong, as oxygen is needed to make something burn, and there is no oxygen on the asteroid. (01:27:40)
Continuity mistake: When Hallie wants to get a popsicle/lemonade, her hair bobble is green, but in the next shot it is white and then changes back to green again. (00:24:30)
Continuity mistake: After Kathleen (Meg Ryan) and Joe (Tom Hanks) talked about what it would have been like if they hadn't been enemies in the book business, Kathleen says she has to go and heads off to her apartment. During this time, the camera switches back and forth between Joe and Kathleen. Kathleen's right sweater sleeve cuff is by her wrist in one shot and rolled up by her elbow in the next. In the next shot it goes back down again. (01:47:20)
Revealing mistake: When Puffy the dog bites Ted and pulls him along, Ted digs his fingernails into the floor, they scrape along, making marks. If you look very closely you can see faint pre-made marks made in the floor, where he drags his nails along. (01:28:45)
Continuity mistake: After Chris is dead, and his wife is writing in her diary, and she writes "I still exist" as she's writing it, exist is written "EXST", then when she looks to read what she wrote it reads "EXT". The writing's different too. (00:22:10)
Continuity mistake: When Alejandro and Diego fight in the cantina, Alejandro knocks over stalks of corn rolled up in a bundle into the doorway. In the next shot, the corn stalks are standing again. (00:35:30)
Continuity mistake: Vic is drinking at the bar and talking about his guilty conscience - or lack thereof: he holds his glass with his right, left, right depending on the camera angle.
Deliberate mistake: While Laurence is driving to the park trying to find Martha there are closeups of his car en route, and one of the closeups of the front left side of his car is a flipped shot, note the backwards "Saab" and the surface damage which is actually at the right side of his car. (00:56:00)