While watching Charlie play blackjack, one of the casino security officers says that "no one can count through a six deck shoe". In fact, this is quite possible for someone with just basic math skills to do using the "hi-lo" counting method. A security officer of any major casino should know this.
When Raymond is insisting that he get his boxers at K-Mart in Cincinnati, he repeatedly states that K-Mart's address is 400 Oak Street. That, however, is the address of the Vernon Manor, the hotel at which Raymond, Charlie, and Susannah stay.
When Charlie and Ray first arrive in Las Vegas, they are traveling southbound on the Strip as evidenced by the Westward Ho passing left to right on Ray's side of the car. A few shots later they are passing the Flamingo Hilton in a northerly direction. Finally, a shot facing south shows the car again passing the Westward Ho in a southbound direction.
At Charlie's house, Raymond opens the freezer door to get some frozen waffles, then goes to the toaster oven and turns it on despite the fact that he did not put anything in it. Yet, a mere 30 seconds later, smoke is coming out and sets off the smoke detector, not nearly enough time to burn anything, let alone even warm up.
In the moments after Charlie pulls away from the diner and the playing cards fly into the air he begins to explain casino rules to Rainman. As they drive toward the camera the sun casts a shadow to the left of the screen. In the immediately following reverse shot as the car drives away from the camera and the conversation continues the sun casts a shadow in the opposite direction.
When Charlie gets out of the car and tries to tell Raymond it's not funny when he does "Who's On First," Charlie carries his jacket in his right hand. It then cuts to a closer shot and Charlie raises his right hand, which is empty. The jacket is later seen in his left hand, but he didn't have time to shift it there.
When Raymond is hears the strange noise in Charlie's room, we see bright daylight through the curtains in both rooms. Upon being brought back to his room shortly after, Rainman remarks that it is several minutes before 11pm (bedtime). When Charlie turns off the lights for Raymond to go to sleep, the room is totally dark.
In the restaurant scene when Charlie causes Raymond serious injury, the pack of cigarettes on the table changes. It starts with the pack being normal, then it shows it slightly crushed and then it returns to normal.
Just before Raymond gets on the train, the woman who passes by him and Charlie on the right wearing a light blouse and jean shorts gets on the same train twice; once after passing Raymond and Charlie, and then again just before Raymond gets on the train.
When Charlie, Raymond and Susanna are in the casino on the way to Raymond's date, just before Charlie is asked to see head of security you can see in the mirrored pillar the reflection of a woman motioning extras to walk past the camera.
From the time they left the farmhouse in western Oklahoma, they supposedly passed some country with mountains in the background (west) between there and Amarillo, Texas. There are no mountains in that part of western Oklahoma (though some small ones exist farther south near Lawton and Altus) - and the nearest "mountain" on that route would be Tucumcari Mountain.
Laundromat scene was supposed to be in Tucumcari, N.M. The town of Tucumcari is larger and has no tall, red-rock mountains in the background. It's on the edge of the high plains, but does have some small mesas west of town.
While crossing the Suspension Bridge in Cincinnati (the 'singing bridge' that Raymond hums along to), the traffic in the background suggests that the bridge is either 4 lanes, or a one-way street. In fact, the bridge has only 2 lanes, one northbound and one southbound.
When Charlie and Raymond are leaving Cincinnati the sun is still up. Later, when Raymond gets out of the car on the side of the highway, it appears that the sun has been down for a few hours. At this point, a sign for Interstate 275 is visible on the side of the road. I275 makes a large loop around Cincinnati area, which would suggest that they had been driving in a circle for a few hours.
"The People's Court" and "Wheel of Fortune" are syndicated TV shows, and, therefore, air at different times of the day in different TV markets - but Raymond is able to watch them at the same time each day (a change in the TV schedule would have been traumatic for him) during his cross-country trip.
Raymond tells Charlie in the bathroom on the day he (Rain Man) was taken from his Ohio home that there was 7.2 inches of snow ("very snowy that day"). Yet, according to the Farmer's Almanac, there was no precipitation anywhere near Cincinatti on that day, which reached a high temperature of 45 degrees.
When the Lamborghinis are being unloaded from the ship the cars have the normal 8.5 x 15 inch front and 12 x 15 inch rear wheels already installed rather than the plain, narrow wheels the factory installs when shipping the cars overseas.
WOXY, "97X, Bam! The Future of Rock and Roll," was a small, independent radio station located in Oxford, Ohio, about 30 miles north of Cincinnati. Clear reception would have been practically impossible on the Southeast side of Downtown.
According to the EPA's 420 publication, Importing Vehicles and Engines into the United States, in 1998 the EPA allowed owners a one-time exemption for importing their foreign cars into the U.S. The EPA does not meet car buyers at the docks, and the idea that Charlie would have had to have the engine modifications right there is ridiculous. Charlie could have easily gotten his one-time EPA waiver for his four Lamborghini's; thus, his having to pay a $10,000 modification for each car before leaving the dock is completely without merit. In the worst case, at the time Charlie could have taken delivery of the vehicles and sold them to the prospective owners, the latter of which was THEIR responsibility to follow EPA guidelines, since Charlie was only a middleman, a broker, which could have also had any required modifications deferred to a later time. Consequently, Charlie's constant and frantic phone calls back to his home base about the cars "becoming legal" were totally unnecessary.
At the end of the movie, Raymond is seen boarding the Amtrak train Desert Wind in Los Angeles. The train is shown to have a single-decker Amfleet consist. The Desert Wind was actually a Superliner double-decker train.