Revealing mistake: At the start of the movie it's night and raining, but you do not see any drops of water hitting a surface anywhere although/but things appear to be wet.
Visible crew/equipment: In the non-widescreen version, a hole in the ceiling and a microphone can be seen picking up the dialogue between Patrick McGoohan and Clint Eastwood in the warden's office.
Character mistake: In the scene where the warden gives Frank the rules of Alcatraz. The warden states that prisoners do not get to see newspapers or magazines. Later when Frank distributes reading material from the Library to D block, he offers the prisoners books or magazines. One prisoner is seen taking a magazine to read.
Continuity mistake: In the scene where Frank Morris arrives at Alcatraz, he has to take all his clothes off for a routine check. When the guards escort him to his cell you can see him wearing a pair of white boxers, but in the next scene (still walking) he is naked again and when placed into his cell you can see him wearing a pair of pants.
Factual error: In the opening scene that shows San Francisco scenery there is a shot of a large TV tower called Sutro Tower. The film takes place in 1960 and Sutro Tower wasn't built until 1973.
Deliberate mistake: When the inmates first leave their cells we see at least 2 that have white or light colored shirts on instead of prison blue.
Factual error: The movie is supposed to take place in 1960, but the oscilloscope on the metal detector outside the wood shop is a new 1979 model, the year the movie was made.
Revealing mistake: Frank is put in Block-D. Then one guard comes and slides open the door, and light pours onto Frank (who is curled up in the corner). It goes to a shot of the guard standing behind the bars, which he has not yet slid open. But in the previous shot when the light was on Frank, there were no shadows cast from the bars - there should have been.