Factual error: Towards the beginning of the film, the Cashes receive a notice about their rent. It's the 1950's and the address has a ZIP code. ZIP codes were not introduced until 1963.
Revealing mistake: In the scene of Johnny's first audition, the record producer is sitting at a table telling Johnny he wasn't believable as a God fearing man. On the table is a pack of cigarettes with a bar code on it. Bar codes weren't in use until 1974.
Factual error: The scene where Johnny is walking home from June's house at dusk. If you look at the phone pole as the camera pans you will see cable TV equipment which certainly wasn't around at that time.
Continuity mistake: In the opening scene it shows Johnny Cash spinning the saw in the prison's wood shop with a glass of water beside the saw. The water is very much clear in color, but later when they go back to the scene the water has a yellow tint when he throws it on the stage.
Continuity mistake: In the scene where Johnny Cash and June Carter first sleep together, watch Cash's hands as Carter goes to answer the phone. In the first cut his left hand is on her bottom and his right hand is on her lower back. When the shot cuts to show Carter's face his right hand is now below her neck and his left is hand is no longer on her bottom.
Continuity mistake: Watch Johnny's shirt collar during the scene where he first plays for Mr. Phillips. As Johnny pleads his case the collar moves, from shot to shot, from under the guitar strap, to over, then under, and over again.
Other mistake: In the scene where Johnny Cash is seen beating his first wife, there are at least two shots from behind where the "tag" on Johnny's t-shirt can be seen. However, this visible tag is one of the new Hanes tag-less tees, which clearly was not available at that time.
Factual error: When the song "Walk the Line" is first being played in the studio during the montage, the guitar player hits a high note on the E string (the thick top string). His fingers were not low enough to play that high of a note on the E string.