When Margot Robbie (Sharon Tate) was talking to the girl at the box office of the movie theater in Westwood, you can see the Starbucks sign for half the scene before it was covered up. Starbucks was founded in 1971.
When Cliff drives home on the freeway and just before taking the off-ramp there is a clearly visible numbered exit sign. California was one of the nation's last holdouts and did not use numbered exit ramps until 2002; the movie takes place in 1969.
In the movie Sharon Tate and Roman Polanski attend a party at the Playboy Mansion. Although there was a Playboy Club in LA in 1969, Hugh Hefner did not buy the Playboy Mansion until 1971 making Tate's attendance rather difficult.
The watch Cliff can be seen wearing throughout the film is a Citizen Challenge Timer, aka Bullhead. Despite the film being set in 1969, the movement found in this watch wasn't released until 1972, with the rounded variant Cliff wears following slightly later.
When Dalton first meets the little girl actor and sits on the set porch with her while she's reading a book, he pulls out a pack of Parliament cigarettes and lights one up. The Parliament pack had a much different design in 1969. The one shown came out many years later.
In a scene set in February 1969, Marvin Schwarz warns Rick Dalton that he might end up playing a villain on an episode of "Batman". The "Batman" TV series had already been canceled in 1968, before the events of this film.
Way too much swearing for 1969, especially in public. Anyone shouting "fuck" in Musso & Frank's would have been politely asked to tone it down or leave, regardless of how important or famous they may have been.
While Brad Pitt and Leonardo DiCaprio are driving early in the film, Joe Cocker's version of "The Letter" is heard, presumably coming from the car radio. The record was not released until April 1970, more than one year after the action of the film first takes place.
The first time we see, and hear, Cliff Booth driving off from Rick Dalton's home, in Cliff's own car, a vintage VW Karmann Ghia, he drives off with the car's engine in high gear. The sound we hear however seems not to be that of a the typical air-cooled engine VW motor, but more of that of a sports care with a conventional engine.
In the Spahn Ranchhouse, after Cliff Booth (Brad Pitt) enters, on a table against the far wall is a famous bronze sculpture (by Remington, I believe). It is facing to the right. A minute or two later it is facing to the left.
When the family member falls in the pool, towards the end of the home invasion / fight scene, you see blood pour out of her into the pool, but when DiCaprio gets the flame thrower and comes back there is no blood in the pool. Also, since the woman was in the pool and hit with the flame thrower why wouldn't she just drop under the water to put the fire out ?
Tarantino is known for adding deliberate continuity errors in this one, as James Stacey comes over to talk to Rick on set, he is bare headed as his cowboy hat hangs on this back. There is a sudden edit jump and now the hat is on his head.
The rear view mirror of the Cadillac disappears and reappears from shot to shot. (This is common in many productions, for obvious reasons, especially for camera shots from the rear of the car looking forward, as is the case here. Production also often remove headrests, although in this movie most of the cars predate headrests.)
Cliff is given a cigarette said to have been dipped/laced in acid (LSD), but smoking anything dipped in LSD would not work because when lit on fire the fire would destroy the LSD. LSD can not be smoked, it only works when eaten/or when liquid LSD is dropped in the mouth, or very very powerful LSD can sometimes be absorbed through the skin.
PCP and embalming fluid are the drugs that work when dipped, dried, and then smoked. People would dip/lace a cigarette in PCP or embalming fluid and then sell them on the street.
When Rick, Francesca and Cliff are returning from Rome, they are seen in one of the tunnels that were used to connect the gates at LAX to the curb. (Currently, the tunnels are closed to the public and are located two levels beneath the current-day terminal concourses.) Cliff is seen pushing a cart fully loaded with their luggage. In reality, then, as now, baggage claim was located by the curb so they couldn't have had checked baggage while in the tunnel.
When showing the 747 model in at least one of the flight scenes, the wrong model was shown. A model of a 747-800 series (with some of the windows edited out) was used. The 800 series didn't fly until the 2000's.
When the date shown is a Sunday in February, KHJ is playing on the car radio, with Robert W. Morgan on the air. Morgan didn't work on Sunday (usually), as he was the morning drive jock Monday through Friday.
Incorrectly regarded as a goof. The generally accepted "Golden Years" of Hollywood did end in the 50's as mentioned in several goofs, but this is Quentin Tarantino's vision of Hollywood. As someone born in the early part of the 60's, especially given his known love of 60's and 70's B movies, this would have been his Golden Years.
A bus drives past showing an ad for the show Combat!. That show went off the air in 1967. This incident is probably not a goof. The ad also said it was being shown back to back on Channel 11. At that time, Channel 11 was an independent station and their schedule was filled with reruns, of which Combat! may well have been one.
Incorrectly regarded as Goof: The item on LSD-dipped cigarettes not working to give a high. The LSD-dipped cigarettes were sold on the street (in broad daylight) by hippies looking to make a buck (or in this case, 50 cents). They could care less whether they actually worked. The cigarettes could have been dipped in anything; saying it was LSD as a new way to get high was a sales trick. They just wanted the money.
When Charles Manson visits the Polanksi house he is told that Terry no longer lives there. One this didn't actually happen, but had it,why would he send his "family" to kill Terry and everyone in the house when he knew they weren't there.
Regarding numerous comments listing the film's entire premise as an error: The Golden Age of Hollywood continued after the 1950s. What ended were, first, musicals, then Westerns. After that, many blockbuster productions continued, with legendary stars from 1930s-50s Hollywood, such as The Towering Inferno, Airport, Earthquake! and The Poseidon Adventure. not to mention numerous war-themed films. In addition, the Studio System, where stars were signed under contract to a specific studio, continued until 1984.
When Cliff is driving the Caddie, just before he sees Pussycat for the third time and picks her up, the speedometer on the car can clearly be seen at zero despite the fact he's driving down the road, revealing the process shot.