Generally, the flap of the piano must be opened while its lid is open.
Greta Dunwoody's piano has its flap open correctly, seen by Holmes and Watson as they arrived (36'22'').
But when the visitors approached her to have a chat, with the camera switches to shoot her from the left side of the piano, the flap is closed, while the lid is still fully open.
This is not only the error of continuity but also unsuitable, for it might damage the hinge of the flap, and would make some scratches on the rim.
Current knowledge does not allow for deliberate changes to human DNA. It has been tried as a cure for some genetic illnesses, but so far without success.
After Holmes and Watson are fired from the case, Holmes brags about how he had stolen Lydon's comb to get a DNA sample from the hair on it. However, he holds the comb with his bare hand, which would contaminate it with his own DNA and would make any test result from it unusable.
This would be true if they needed DNA from something else on the comb, but Holmes states that he stole it to obtain a hair sample. A hair on the comb would not be contaminated just from touching it.
Moreover, mixed DNA samples are routinely separated, otherwise, samples from victims to obtain the DNA of perpetrators such as in rape kits, for example, would not exist.
When riding in the limo, the background scenery is shown running in reverse for one of Sherlock's shots.
The doll clearly does not "melt" from the "acid" Holmes pours over it. The face collapses because the air is sucked out.
Early on, when examining a murder victim's wallet, Holmes handles a New Jersey motor vehicle registration card with a renewal sticker attached. New Jersey only used stickers from 1999 to 2004, and never resembling the California design shown here.
Sherlock calls the bee "omnia avoseta." The correct term is "osmia avosetta." Sherlock uses the correct term the second time he refers to the bee.
At one point, Holmes claims he cannot discern the chemical formula because the elements are not labeled. However, the convention for this type of formula is that unlabeled elements are always carbon, and hydrogen atoms are always omitted since they can be taken for granted, which he should know.
Holmes wears ancient welding goggles when pouring the "acid" on the doll. While eye protection is an essential precaution when handling dangerous chemicals, this is the worst conceivable kind. He can hardly see what he is doing, especially since the room is only very dimly lit.