The "fez" hat worn by several Turkish men in the movie has
been extremely rare in Turkey since 1925 when it was abolished by Kemal Ataturk (the leader and founder of the Republic of Turkey) in an attempt to "westernize" the country.
The Turkish spoken by the Turkish characters in the film is uniformly broken. The actors are obviously not Turkish; sometimes the language is so broken it is difficult for native speakers to understand what they are saying.
Billy Hayes never had any real issues with the Turkish people, and made some friends in prison. He did not have any profanity rant when the Istanbul Appellate Court forced the trail judge to give him a minimum sentence of thirty years, but actually begged for forgiveness when he spoke before receiving his new sentence.
It is true that Billy Hayes was beaten with a stick by a guard for stealing a blanket because he was cold in his jail cell that first night. He was hit with a stick, but was not hung up and beaten on a torture rack as shown in the movie.
Several of the month and date cards shown in the movie are very inaccurate, especially toward the last third of the film. The movie deploys Hayes decent into almost a complete vegetable, based on how the Turks treated him. In Billy's Midnight Express book, he does not suffer the mental hardship and physical abuse as shown in the movie. In contrast, the book talks more about the filthy conditions at the prison, not Billy's decaying health, nor any issues with the people of Turkey.
During the opening scene, in the bus that takes Billy and Susan to the plane, Susan is reading a newspaper and says "Janis Joplin died yesterday". Billy was arrested at Istanbul Airport on October 7, 1970. Janis Joplin died on October 4.
After Billy Hayes pushes Hamidou against the wall, killing him, he is seen escaping the jail dressed in well fitting prison guard clothes. Hamidou was much taller and heavier than Billy Hayes and the guard's clothes would have been baggy and loose fitting. The location where Hamidou had taken Billy Hayes was secluded with no other guards or clothing nearby.
When Billy, Jimmy, and Max are trying to escape through the catacomb prison tunnels, they are joined by a fourth inmate wearing a hat, who has some small dialogue. But, when the inmates discover that the escape route is a dead end, the fourth inmate on their journey is not with them and is never seen again. This fourth person is not mentioned by name, so who he is remains unknown.
The goof item below may give away important plot points.
The plate on the police truck carrying Billy to prison after he's caught trying to escape has four errors. First, the blank between the alpha characters and the numeric characters that follow is missing. Second, in the year 1970 when the movie is set, plates in Istanbul had not yet reached YY (they advance chronologically from AA to ZZ). Third, plates in Istanbul are framed with a thick line of a color the same as the letters - the plate in the movie has no frame. Four, most seriously, police vehicle plates have white characters on a black background - in the movie, the plate is just the opposite.