Goofs

When Doctor Watson is pulled out of the mire by Stapleton and Cecile, he is covered in mud up to his shoulders. However, in the next scene, when he gets off Stapleton's cart at Baskerville Hall, the front part of his jacket (not covered by the blanket) is completely clean.

In the prologue, after Sir Hugo has stabbed the girl on the moor, there is very little blood on the knife, but after he has been attacked by the hound and drops the knife, a close up reveals it to be covered in blood.

In their discussion regarding the source of the tarantula used to attack Sir Henry, Watson asks Holmes how he knew the spider had not secreted itself with Sir Henry's luggage from South Africa and instead came from the collection of a local and eminent entomologist, Bishop Frankland. In classic form, Holmes says, "Elementary, my dear Watson, tarantulas are not from South Africa." He is wrong, as tarantulas, such as the baboon spider, are native to South Africa. A bit earlier in the film, Bishop Frankland asks if the tarantula in question had originated from one of the village. Here the expert was mistaken as tarantulas are not native to the countryside or villages of England. (To be fair, the good clergyman may have been trying to avoid admitting that a tarantula loaned to him by the London Zoo had gone missing.)

Although they look fearsome, tarantulas are not deadly spiders. Their bite is no more harmful than a bee sting. Sir Henry's life was not in danger.

EDIT: Correct. However, it is clearly stated in the film that while tarantulas are not harmful to the normal man, Sir Henry's heart condition means that his life IS risk from the tarantula, either by physical means, or from the stress on his heart caused by arachnophobia.

After Stapleton is shot, the dog starts to run past him. Stapleton clearly pulls the dog onto him to make it look like he is being attacked.

The window pane in the Bishop's room is already cracked (presumably in order for it to smash in a certain way) before it is broken by the telescope.

When Sir Hugo lets loose his hunting hounds to pursue the servant girl (at around 5.40 mins), their barks are obviously a recording that is played repeatedly.

When Sir Hugo is chasing the servant girl across the moors, a day-for-night style of photography is used. However, the shots are inconsistent, and range from semi darkness to full clear daylight.

The Spanish saying that Cecile mentions is "hierba mala nunca muere" which she translates as "weeds are not killed by the frost", but the actual translation is simply "weeds never die".

When Sherlock Holmes is searching for a map of Dartmoor in his study, the pile of papers on top of a bookshelf contains a pile of modern box files, one with the label showing. After a cut, the box with the label disappears - so someone probably noticed.

The chessboard is set up incorrectly (the rightmost square on the first rank is supposed to be white, not black).