Continuity: 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.
Revealing mistakes: The window in the bishops room is already cracked (presumably in order for it to smash in a certain way) before it is broken by the telescope.
Revealing mistakes: After Stapleton is shot the dog starts to run passed him, Stapleton clearly pulls the dog onto him to make it look like he is being attacked.