Errors made by characters (possibly deliberate errors by the filmmakers): On the witness stand, Janet McKenzie refers to October 8 as being one week to the day before October 14 (the night of the murder). It's only six days earlier.
Anachronisms: Though made in 1957, the film takes place in 1952, but in the opening shots of Sir Wilfrid's car on the streets of London (and in the rear-projection shots in the car's interior), several post-1952 cars can be seen in the background.
Revealing mistakes: When Vole, in his cell, starts relating the story of how he and Christine met, the scene dissolves into a flashback while Vole is speaking. But as it begins to dissolve, Vole, still clearly visible, has stopped talking even as his voice-over continues its narration.
Incorrectly regarded as goofs: In the first courtroom scene, the clerk twice states that the murder of Emily Jane French occurred in "the county of London". The County of London was known to both Sherlock Holmes and Horace Rumpole. It was run by the London County Council from 1889-1965, was comprised of over two dozen boroughs (Hampstead to Greenwich to Chelsea), and much to Leonard Vole's chagrin, was home to the Central Criminal Courts, the Old Bailey. In 1965, the County of London became the larger, Greater London, which it still is. Administered by the Greater London Council from 1965-86, it has been run since 2000 by the Greater London Authority, headed by a directly-elected mayor and assembly. For reasons of tradition and vested interest, the old City of London, now a square mile of banks, brokerage houses and the Tower, remains a separate entity.
Factual errors: The witness Janet McKenzie testified that October the 14 was a Friday. The film is set in 1952; if the year of the murder was 1952, October 14 was a Tuesday or if 1951, October the 14 was a Sunday. The last year October 14 was on a Friday before 1952 was 1949.
Errors made by characters (possibly deliberate errors by the filmmakers): Both Sir Wilfred and Mr. Myers introduce evidence directly to the court themselves, which is against courtroom procedure. Sir Wilfred, for example, trumps Inspector Hearne's testimony by directly revealing Leonard Vole's blood type, instead of properly calling a witness with personal or professional knowledge of it.
Revealing mistakes: When the photographer takes the pictures of Leonard Vole in the prison, he takes one from the front view and one from Voles right side. Later in the courtroom the prosecutor Mr. Myers shows these pictures to Leonard Vole in the witness stand. Now there is one from the front sight and one from his left side. It is hard to believe, that a professional photographer in a murder case would make such a serious mistake to twist the negative mirror inverted. It more looks like a mistake of the prop master.
Continuity: Early in the courtroom scenes witnesses are seated in the hallway outside the courtroom door on a bench. This has a backdrop painted to look like a longer hallway. At the end of the trial when the courtroom is emptied the hallway is shown as being narrower with no bench.