Corrected entry: In the scene when the doctor asks Raymond unbelievably far-out mathematical equations, Raymond answers them correctly. A few seconds later, he is asked rather simple math problems and is way off with his answers. It seems uncharacteristic that Raymond (being a high-functioning autistic, as the doctor described him, and who also stated himself that he was good with numbers) would be able to answer such technical math equations with ease but not be able to subtract 50 from 100.
Corrected entry: When Charlie sees a small-town doctor to get some advice how to deal with Raymond, the doctor asks Raymond to do some mathematical stunts. He multiplies 312*123 and 4343*1234 without thinking, which is unbelievable enough even for an autistic savant, but the square root of 2130 with 8 decimals after the dot, again without a bit of thinking, is certainly too much. (00:59:50)
Corrected entry: When he's frightened at the airport, Raymond states "Qantas never crashed." Between 1927 and 1951 eight Qantas aircraft crashed and one was shot down by Japanese fighters. A total of 70 people lost their lives. In its modern incarnation Qantas has had a few near misses but no fatalities, but nonetheless Raymond is incorrect, and an idiot savant with perfect recall would not make a mistake like that.
Corrected entry: Not too long after Charlie and Raymond leave Cincinnati by car they come across a bad accident. Raymond gets out of the car and refuses to continue driving on an interstate. According to a sign they are on interstate 245 WEST, which a) I couldn't find on a map in the Cincinnati area and b) shouldn't exist anyway as all interstates with odd numbers go in north-south direction. (00:48:00)