Factual error: Tom Hanks refers to an Intensive Care Unit, and although a few physicians had already discovered the practice of separating the sickest patients into a separate ward in the 1930's, the term Intensive Care Unit was not yet in use.
Continuity mistake: When William "Wild Bill" Wharton spits the Moon pie into Brutus "Brutal" Howell's face you can see that the Moon pie lands on his shirt, tie, lapels of his coat and on the right shoulder of his coat. In the next shot when Brutal and Paul throw Wharton into the padded room, you can see that the right shoulder of Brutal's coat is completely clean, but the Moon pie is still on his shirt, tie and lapels.
Continuity mistake: When they tape Percy's mouth shut before putting him in the restraint room, the handkerchief is partly outside his mouth and gets stuck to the tape, when they let Percy out, the handkerchief has moved. If he's been locked in there with the straight jacket on, there is no way he could have moved it.
Factual error: When Paul returns home, his wife asks him what the doctor said. He replies "...gobbledygook mostly." The trouble is, the word "gobbledygook" wasn't coined until 1944. This scene takes place in 1935.
Continuity mistake: The guards take John to see the Fred Astaire movie and John sits well to the right of centre in the theatre. We see John mesmerized by the movie, Paul sitting a row behind in the centre of the seat theatre and the light from the projector spreads wide directly behind and over top of Paul's head. Yet, when we see John again a moment later, when he's says "They's just like angels", the projector light is directly behind John, deliberately giving the illusion of a halo. This would not occur as shown unless John had moved over to the centre of the theatre.
Factual error: The movie is set in 1935 Lousiana, and all of the executions are through electrocutions. However, executions by that method in that state didn't happen till 1941.
Continuity mistake: Roll on 1 means to crank up the generator all the way. On top of the machine are light bulbs that all light up when the generator is turned up. However, when they are executing Del they have a sideshot of Brutus and you can see the lightbulbs all lit up, but Percy hasn't said Roll on 1 yet.
Revealing mistake: Just before John Coffey is about to be executed, he's sitting on his chair in the cell. As he stands up, you see him lift his leg onto a platform. The platform was used to make him look about a foot taller than he actually is.