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.
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.
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.
Johnny is shown touring with Elvis, Jerry Lee Lewis, and June Carter 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 was already recording for RCA, and touring on his own.
When Johnny Cash 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.
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.
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.
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.
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 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.
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.
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.
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.
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.
When Johnny Cash is playing at the prison, a pair of large speaker cabinets are visible on the stage. These speaker cabinets are model Yamaha 4115 which were not produced until the mid-70s / early 80s.
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.