Johnny is shown touring with Elvis, Jerry Lee, and June for Sun Records early in the movie. In fact this could not have happened. By the time Jerry Lee Lewis was signed to Sun Records. Elvis Presley was already recording for RCA, and touring on his own.
In one mid-1960s scene, Waylon Jennings has long hair and a beard. At the time, Waylon was clean-shaven, with short, slicked back hair. He adopted the beard and long hair in the mid-1970s "outlaw" era.
In the variety store in Wheeling, W.V. in 1958, June picks up a fat white candle and sniffs it. Candles of that era were almost always tall tapers, unless they were the school-made variety you created by pouring wax into a waxed-paper milk carton, and gave your mama for Christmas. Either way, they were never scented.
When Johnny first looks into Sun Studios, and sees Elvis Presley recording Milk Cow Boogie, Elvis is playing with a guitarist, bass player, and drummer. While with Sun records, Elvis's band consisted of Bill Black on bass and Scotty Moore on lead guitar, with Elvis himself playing rhythm. Drummer D.J. Fontana didn't join them until Elvis signed with RCA Victor and went north.
Jerry Lee Lewis, who sings right before Johnny Cash in the first show, wasn't famous at the time. When Cash plays in Walk the Line for the first time, he'd only had one hit, and was clearly not used to the stage.
In an early scene, Johnny and Jack Cash walk down the road on their way to the fishing hole. Johnny quotes a line from a Foghorn Leghorn cartoon. The scene takes place in 1944; The Foghorn Leghorn debuted in 1946.
When Vivian shows John the eviction notice, the document shows an address with a ZIP code. In the 1950s, a city the size of Memphis would've had zone numbers (ex. "Memphis 11, Tenn."). ZIP codes started in 1963, and they weren't routinely used in places with one ZIP code for several years after that.
When Sam Phillips tells Johnny about singing "the one song you would sing if it were your last", a shelf in the background to the right of Sam (slightly out of focus) has vacuum tubes sitting on it. A large box with the 'National' (Richardson) logo is in the middle of the stack. That logo wasn't used until the 1970's.
After Johnny watches the shoe shine boy "get rhythm", he turns to walk across the street to Sun Studios. He is standing in front of a 1956 Ford station wagon with American Torque Thrust wheels. The scene takes place in the mid 1950s; the wheels were introduced in the mid 1960's.
When Johnny is talking with Vivian on the phone from Germany, an apparently US military aircraft is visible through the window. Although most of the markings appear to be correct, the fuselage has an FAA-assigned alpha-numeric N number. US Military aircraft designators are all numeric, and displayed on the vertical stabilizer.
When Johnny walks back from the Carter house, he is near a railroad crossing. The scene is set in the 1960s, but modern crossing gates are shown. In addition, a signal shanty with a white light on the front is to the right of the crossing. The shanty didn't exist in the 1960s. The white light, a sign that the power is on, was introduced in the 1990s.
Early in the film, Johnny Cash is seen walking in uniform in Germany and the year 1952 is shown. He sees a newspaper story announcing the death of Hank Williams, Sr. Hank Williams, Sr. died on January 1st, 1953.
When Johny and June are leaving the store with the fishing poles in Wheeling, WV, June says "I think there's a lake this way". There are no lakes near downtown Wheeling; however, the Ohio River runs along the entire length of the city.
When Johnny is seen playing with his band for the first time on his porch, Marshall Grant's bass is marked with tape as fret markers with the notes written on them. But you can see there's a whole step between "B" and "C". Any musician would know there is only a half step between "B" and "C" and "E" and "F".
When Johnny and June are in the store in Wheeling, WV, the thick southern accents of the locals are not accurate to the region. In reality, the Northern West Virginia accent found in Wheeling is very close to a standard American accent.
When June goes to the theater one morning on the tour to meet the boys, she finds they've partied all night and she becomes angry. In the shot of her approaching the stage, only 3 bottles are within reach. When she throws them, she throws 4 bottles.
While Johnny and June sing the duet "Jackson" onstage, June's hairdo changes during the scene. In the beginning June's hair is parted down the middle with long bangs tucked behind her ears. When June sashays to the side to allow Johnny a solo moment to sing his part, just a few moments later she sashays back with an elegant upswept hairdo which remains through the end of the song and scene.
Before singing at Folsom, Johnny is touching the blade of a table saw with a glass of relatively clear water next to it. Later when he holds the glass for the crowd, the water has turned dirty and yellow.
When June is throwing the beer bottles at the guys, in the long shot, Johnny jumps behind the fallen table, and there is nothing in front of it. In the close-up there is a chair in front of the table. When June leaves, the chair is gone again.
Johnny and June are lying in bed in the hotel room when the phone rings. June picks up the phone with her right hand, and starts to put to her right ear. In the next shot, the phone is in her left hand and at her left ear.
At the show in Texarkana, there is a Purina advertising sign on the backdrop of the stage. In some scenes, it says "Purina Chows for Poultry and Livestock", but in other scenes the words Purina Chows has been removed.
The "US" collar brass (in-the-circle) seen properly on the collar of the long-sleeve khaki shirt, was not worn on the left side of the overseas cap. Anyone doing so would have been out-of-uniform and subject to disciplinary measures.
"Ring of Fire" was written earlier than 1965. Anita Carter released it in early 1963, and Cash released his own version of it later that same year. It is shown as being written after 1965 by the film's chronology.
After Johnny leaves June's house after walking from Nashville, it rains later in the scene. Johnny passes out and falls on wet grass and mud. The next morning, when he wakes up near the house he eventually buys, his face and white shirt are clean. They should have been dirty from wet grass, if not muddy.