Revealing mistake: Whenever Stupid Bat is flying, the wires pulling him up are visible.
Continuity mistake: Lady Britt Dorset tells Mr. Graham to hop on her new car. He sarcastically takes exception to the car being 'hers', since she paid for it with the money he gave her. Between these two shots, his umbrella does a 180°. (00:10:50)
Visible crew/equipment: When Mike and Susan prepare to start putting pieces of snow in the kettle, in the shot where Susan hands Mike the kettle, and he begins taking the lid off, the camera crew can be seen reflected on the kettle. (01:16:00)
Other mistake: Like most Sherlock Holmes films 'The Private Life Of Sherlock Holmes' is set in Victorian England: Queen Victoria even makes an appearance. Holmes and Watson go to Loch Ness in Scotland, where they see the Loch Ness monster. (Spoiler alert) it turns out that the Loch Ness Monster is not a living creature, but an experimental submarine. Like most people who would have seen the film on its release in 1970, they are familiar with the Loch Ness monster (even if they do not necessarily believe in it). But the first documented sightings of the Loch Ness Monster were only made in 1933. Nobody ever thought there might have been a monster in Loch Ness before 1933.
Suggested correction: Sightings and lore of the Loch Ness Monster date back over 1,500 years. In fact, the indigenous people of the region carved images of the monster into stone as far back as 500 AD. The 1933 hoax was certainly not the first time the monster was sighted; however, the hoax was inspired by the centuries-old Loch Ness legend, of which Holmes, Watson and everyone else would be well aware in the Victorian era.
The only carved images from that period are Pictish symbol stones, none of which are particularly associated with Loch Ness.
On the contrary, the Pictish "Drumbuie Stone" (recovered at Drumbuie Farm on Loch Ness in the mid-19th Century) depicts a large serpentine creature, very much matching traditional descriptions of the Loch Ness monster. Https://canmore.org.uk/site/12626/drumbuie.
Suggested correction: This is somewhat incorrect. The 1933 photograph that was published in newspapers may have brought the idea of a Loch Ness Monster to a wider audience, reports of a creature in Loch Ness (or Loch River) were around long before then. And just because the term "Loch Ness Monster" may have first been printed in 1933 doesn't mean the term didn't exist before then. In a fictional story surrounding fictional events, there's no mistake in bringing up a creature already rumored to have existed.
Well observed sir! I thought somebody might well say that. Maybe I should have gone into more detail. May I make it clear that I have absolutely no problem with a sighting of the Loch Ness Monster in a Sherlock Holmes film, since Sherlock Holmes was a fictional character, and 'The Private Life Of Sherlock Holmes' was an imaginary story. (Plus the film contained some intentionally comic elements, it was a bit 'tongue in cheek', so lets not take it too seriously!) But lets look at the history of sightings of the monster. The first sighting to attract widespread attention was on 22 July 1933, when the Spicers saw a creature near (but not in) the Loch. On 12 November Hugh Gray took the photograph you allude to. In 1934 Rupert Gould published the first book about it. You say that earlier sightings may not have been widely reported. You are quite correct! One D. Mackenzie said he saw a monster in the Loch in 1872, but did not tell anybody at the time. A sixth century life of St. Columba records an encounter with a 'water beast' in the River Ness. My point was that, in the film, Holmes, Watson, and most other people, are familiar with the story of the Loch Ness Monster. (Spoiler alert again) : The 'monster' is an experimental submarine, which Sherlock's brother, Mycroft, is helping the war office to develop. To stop people realising they were experimenting with new military technology, they would develop the submarine in Loch Ness, so anybody seeing it would think it was the Monster (to add to the deception they give it an artificial neck and head). My point is that, while most people who saw the film in 1970, and most people using this website, would be quite familiar with the story of the Loch Ness Monster. So, whether or not they believe in its existence, they would get the joke (after all, the film was not meant to be taken completely seriously). In the Victorian era the Loch Ness Monster would, at best, have been a local rumour, not something that was known worldwide so it is doubtful that even people as undoubtedly intelligent as Holmes and Watson would have known about it. If they saw a monster in Loch Ness they would not say 'Oh, that's the Loch Ness Monster'. They would ask 'Whatever is that great big thing going through the water?'.
Continuity mistake: At O'Malley's release, he is approached by a man wearing a red fez. The people in the background change position at the close-up (the guy in orange turtleneck gets much closer to the one in a blue suit, for instance). (01:09:55)
Continuity mistake: Henry Miller at the beginning of the movie is mooching a few Francs to go out with his wife who just arrived in town. As he is trying to get a few from Boris, Mona's hair keeps changing position in front or behind her shoulders.
Other mistake: The steer horn that penetrates Forrest Tucker near the end of the movie is at right angles to the rest of the steer horn.
Other mistake: After the cricket match when Elspeth Wentworth runs away from her father, Mr Wentworth (Richard Wattis) calls her Kathryn, mistakenly calling the character by the actress' name (Kathryn Dawe).
Continuity mistake: Professor van Helsing and Doctor Seward enter the Harkers' bedroom. A few seconds later, they enter the bedroom again. (00:52:00)
Visible crew/equipment: When the police investigator is speaking with the man standing outside the store window and asks him if he would buy every tie in the store, in every close-up shot of the investigator, the reflection of the cameraman kneeling on the ground can be seen in his sunglasses. (01:08:25)
Factual error: The film is set in 16th century England. In the first scene, when the accused girl is branded, she screams, and reveals a full set of teeth with nice silver amalgam fillings. (00:03:35)
Other mistake: A little more than halfway through the movie, they are robbing the train cars by uncoupling and switching the switch direction back and forth to direct the unwanted engine and caboose to the left branch. He switches the switch to go right for the wanted cars and forgets to switch it for the caboose. If you look at the tracks of the switch, it is already set for the train to go left. He then remembers to change the switch and the caboose goes left where it would have already gone.