Books on the second shelf up from the bottom in Elwood's library change position throughout the movie.
Elwood's hands change position when Dr. Sanderson 'meets' Harvey.
In the daytime scenes at Chumley's Rest, shadows are seen of the actors and props that clearly go against the dominant natural light.
The cab driver comes in to get Veta Louise to handle Elwoods luggage. When the orderly carries her in from the garden, there is no sign of the luggage.
Elwood forgets his bathrobe in Dr. Sanderson's office when he leaves the sanitarium.
When Jimmy Stewart sits down in the library with a Jane Austin book you can see a fly land on the golden edge and then fly off.
No-one can see Harvey, yet someone painted his portrait.
When Gaffney is saying that he will be taking the cab home near the end of the film, his mouth movements don't match his words.
The story takes place in Indianapolis, and Dr, Chumley's car has an Indiana license plate. The mountains outside the sanitarium are clearly Southern California.
Harvey the rabbit is claimed to be 6 foot and 3 and a half inches tall. James Stewart is 6 ft 4 in, yet he keeps looking up to Harvey. This is because Stewart wanted Harvey to be 6 ft 8 in but the script of the original play wasn't changed to reflect this.