Factual error: During the bare-knuckle boxing match, Holmes uses the word "discombobulate" to describe one of his moves. Per Merriam-Webster, this word was first coined in 1916; the movie is set in the early 1890s.
Revealing mistake: In the scene where Holmes is sitting on the chair next to Sir Thomas's dead body, the body's eyelids twitch.
Continuity mistake: At the end of the film, in the bridge confrontation between Holmes, Blackwood and Irene Adler, Lord Blackwood forces Adler to the edge of a precipitous (and deadly) fall into the Thames. There is even a perspective shot, looking straight down, of her heels teetering on the edge and nothing between her and the river except two sailboats far below. There are obviously no scaffolds or other means of obstructing her fall in this shot. It's a dead drop. Then Blackwood shoves her straight backwards over the edge. But, when Holmes looks over moments later, Irene Adler is safe and sound on a convenient bit of scaffolding that magically appeared just a few feet below.
Continuity mistake: When Irene wakes up at the end and says 'I've never woken up in handcuffs before,' her lips have a light pinkish sort of color on them. Then she sits up and her lips have a dark red color on them. Quite a feat, putting on lipstick with your hands cuffed behind you.
Continuity mistake: When Irene Adler turns up at Holmes' apartment, Holmes lifts her necklace and we see the Maharaja's diamond hanging off it. When Watson asks Holmes to explain why he jumped through the window after her, when she leaves, we see another shot of her necklace, meant to be the same shot as before. But in the first shot, the diamond is hanging slightly to the left, and in the second, the diamond is perfectly centred.
Revealing mistake: When Holmes rolls and almost falls off the bridge at the end, it's obviously the stunt double as he catches himself. The double's hair isn't quite as long and you can see it is sprayed or gelled into place, as it doesn't move at all like Robert Downey Jr.'s.
Factual error: The scene in Piccadilly Circus shows the Shaftesbury Memorial Statue, but this film is set in 1890 and that statue was only unveiled in 1893.
Continuity mistake: When the big guy runs up to slipway 5 he passes a group of men and a horse in front of the door. When he opens it the camera is in front of him and behind him is a group of men and a horse and a wall and piles of crates that are nearly touching the horse - these have appeared from nowhere.
