Continuity mistake: In the scene in the restaurant where Tom Cruise and Dustin Hoffman are trying to order breakfast, Hoffman's character fixes the salt and pepper shaker a certain way. In the next shot they are opposite the way he fixed them. Then in the next shot they are back to the way he had fixed them.
Plot hole: Raymond's rigidly set routines force him to watch the TV shows "People's Court" and "Jeopardy" at the same times every day. Traveling on the road, however, this would be almost completely impossible, since the two shows aired in syndication. Not all cities aired them at the same times.
Continuity mistake: In the scene where Charlie and Raymond argue over K-Mart underwear an utterly frustrated Charlie stops the car, gets out, and vents his spleen. There is a close-up of the door opening with a clear vertical shadow, whereas all wide shots suggest that it's late evening.
Continuity mistake: When Charlie has the brainwave to use Raymond's special abilities to make some money in Las Vegas they turn around and drive directly to Caesar's Palace, get dressed up, and make a lot of money. All of a sudden Susanna shows up, and we don't have a clue how she found out that they went there, when, on the other hand, we saw Charlie earlier calling his partner a couple of times, telling him his whereabouts, and even calling Susanna, when it didn't necessarily move the plot forward.
Continuity mistake: In the scene where Charlie Babbitt and Susanna are driving in a car and Charlie gets a phone call, telling him that his father died, you can see through the windows of the car, that they are driving through a town with many houses bordering the street. In the next shot from the outside, when the car is turning over, it's in a desert with no more houses around and a lot of windmills.
Continuity mistake: When Raymond finds Charlie and Susanna making love Charlie runs after him. A moment later we find Susanna soaking peacefully in a hot bathtub, without even nearly enough time to get in. Additionally, when they talk while Charlie is sitting on the side of the tub, wet strands of Susanna's hair appear in front of her shoulder.