At about 80 minutes into the film, the prosecutor cross examines the incomprehensible expert witness. In the first shot, the prosecutor unbuttons his jacket. In the next shot, he unbuttons it again.
Near the end of the movie there is a montage of newspaper headlines, the word PSYCHIATRIST is misspelled in the headline.
When Clitterhouse is locked in the sub-zero refrigerated vault for the furs, his breath should be visible. It isn't.
When Rocks is holding a gun to Dr. Clitterhouse, the phone rings and Dr. Clitterhouse picks it up. He then hangs up after Rocks tells him to put the phone down and let it ring. But the phone continues ringing in the same cadence as if it were never picked up. In reality, it should have made a phone pickup sound. Additionally, once a phone is picked up and then put down again, it kills the ringing. A person would have to dial again for it to ring again.
Dr. Clitterhouse pronounces the word 'Lieutenant' as 'Loo-ten-ant', and pronounces the letter 'Z' as 'Zee'. Since Dr. Clitterhouse has an English accent we can therefore assume that he is an Englishman, and most Englishmen pronounce 'Lieutenant' as 'Lef-ten-ant' and the letter 'Z' as 'Zed'.