Alcatraz was a super max prison, there was no inmate movement except for chow and yard. These inmates walk around like they own the place.
The film is set in 1946, yet one of the guards is seen with a more contemporary, AK-47 type rifle.
The U.S. military officer's apparel, "camo" uniform and even his tinted sunglasses are way too modern for a film set in 1946.
Clarence and Alice talk in the prison through telephones in the visiting area. The receivers are correct, but curly wire was only introduced in the early 1960s.
The calendar on the Governor's desk reads Tuesday 26th April. This could only be true for 1938, 1949, and 1960 around this time frame. The gap is due to leap years. In 1946 when this is supposed to be set, this date was a Friday.