Continuity mistake: When Enola and Tewkesbury approach Basilwether, its lights are on. When they enter, its lights are off. (01:34:30 - 01:35:30)
Continuity mistake: At the end of the film when Enola is deciphering the personals advert she initially writes "Meet me a" but when it goes back to her writing afterwards it has changed to "meet me royal..."
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: When Sherlock says "those kind of mysteries are always the most satisfying to unpick", he's smirking in the closeup, but in the next wider shot from the side he's looking deadly serious.
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)