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 no 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.)