Factual error: The old percussion double barrel shotgun is firing modern shotgun shells, which would never work or fit in that gun, and furthermore are also made of plastic, wrong for the era.
Factual error: At the end of the movie Enola rides a bike on the right side of the street. The shot widens and two horse-drawn buses come into view, also on the right side of the road. The movie is set in exactly 1884 (votation). It became mandatory to drive on the left side of roads in England in 1835 (Highway Act). (01:56:33)
Plot hole: Enola and Tewkesbury make an unpremeditated decision to visit the Basilwether estate. This decision was made on the spur of the moment, and no-one knew about it. but when they arrive, Linthorn, who is supposed to be in London looking for Tewkesbury, is waiting in ambush to kill them. (01:32:45 - 01:34:31)
Factual error: Linthorn meets his end when Enola knocks him off his feet. He hits his temple against a heavy and sharp furniture protrusion. Death must have been instantaneous, but instead, he lives to speak a few words. (01:39:18 - 01:39:55)
Factual error: Viscount Tewkesbury uses Marquess of Basilwether as a secondary title. However, marquess is two ranks above viscount in British nobility; a secondary title would only be of a lower rank, and the only rank lower than viscount is baron.
Continuity mistake: Enola's mother draws the final line through a set of four lines, which are not visible when Mycroft looks at the wall. (00:07:38)
Continuity mistake: The school uniforms completely change halfway through the school portion of this movie. It's the whole top white piece of the dress. (01:19:46 - 01:22:29)
Factual error: Mrs. Harrison travels to a random, remote country mansion (kilometers away from any civilization), meets a woman who is not her pupil, forcibly takes her measurements, insults her, and slaps her! Nobody in the right mind would do that because they know they would be murdered, harmed, or handed over to the police for trespassing, assault, and battery. (Such outcomes are recurrently portrayed in Sherlock Holmes stories.) Schoolmistresses did use corporal punishment but only on their pupils and within the bounds of school, where they have relative safety. To make matter worse, Mycroft warns Mrs. Harrison in advance. He describes Enola as "unbroken", "a wild and dangerous woman", "a wild child." (Indeed she is; she beats people during the rest of the film. How did Mrs. Harrison escape unscathed?). (00:11:43 - 00:13:46)
Other mistake: Inspector Lestrade of Scotland Yard says "I'm a close personal friend of Sherlock Holmes" and "he doesn't have an assistant." Both are incorrect. Doctor Watson is renowned for being Holmes' personal friend, assistant, and chronicler. Lestrade has always been a client or rival, never a personal friend. (00:58:06)
Plot hole: No sooner than the film begins, Mycroft becomes the legal guardian of Enola; Sherlock accepts it without question. How on Earth did that happen? No coroner would grant a transference of guardianship just because someone's mother stepped out of the house and didn't return for a day or two. (00:11:37)