Factual error: During the film Daniel Radcliffe is not seen wearing, or even carrying, a hat on a single occasion. In Edwardian times a man would not have gone outside without a hat on, particularly a member of the professional class.
Revealing mistake: When Mr Kipps comes back in into the house after seeing the ghosts of the children in the front garden, he looks down and sees footsteps. The error is the footsteps are far too uniform and equally spaced at regular intervals to look natural. It looks as though a template has been used to print them on the floor.
Factual error: The driver who first takes Daniel Radcliffe to the haunted house tells him that the tide will be out at 5pm, enabling him to return and collect Radcliffe. A day or two later, Ciaran Hinds says it will happen at 11pm. This is not possible - tide times cannot change by that much in the space of a couple of days.
Continuity mistake: Daniel Radcliffe's facial hair goes from 2 day stubble to clean shaven throughout the movie. About midway through it is very noticeable. He is by himself in the Marsh house, for the 2nd time I believe, and in every shot he has 2 day stubble. However when it cuts to a close-up he has a near-perfect shave.
Revealing mistake: When Arthur goes into the marsh to bring Nathaniel's corpse up, each time he emerges to get air, you can see the lack of mud around his lips from an air mask.