Continuity mistake: The chunks of ice in the asylum dunking scene appear and disappear. When Lawrence is brought to the surface, he has two chucks of ice on him that fall off leaving none, but when the shot cuts to him being raised and locked upright, there's a chunk of ice on his right arm. A minute later when he's lowered again into the tank, there's a large chuck by his hip but vanishes when the shot switches showing Lawrence's left side.
Factual error: Sir John claims to have removed the gunpowder from the silver bullet four bore cartridges. However, he had not removed the primers, thus there would not have been a mere click from the falling hammers but a most definite bang from the primers. Furthermore, in some calibers the pressure from the primers may actually be enough to propel the bullets even without gunpowder.
Other mistake: Gwen regrets getting Lawrence involved, saying that if she had never sent him that letter, he would still be in New York. She sent Lawrence a letter when Ben went missing in the theatrical version but, in the extended cut she went to tell him in person instead. Yet the "letter" line is retained in both versions even though it doesn't make sense in the latter.