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.
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: 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.
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.
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.
Factual error: 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.
Continuity mistake: During the first tour when all the guys are on the stage drinking, June throws 4 beer bottles. The last one is thrown twice because there were only 3 lying there.
Continuity mistake: When John wakes up with Waylon singing, the left side of his collar is rumpled. When he gets up to use the telephone it's straightened out.
Continuity mistake: At the first concert with Jerry Lee, towards the end of his song his white scarf disappears.
Factual error: Another artist on the bill (Buddy Holly I think) in the 50s is playing barre chords on his guitar which were not invented in the 50s.