Other mistake: While Dr. Harvey is looking at his multiple reflections in the mirror over the sink, he sees the Crypt Keeper reflection. It then cuts to a close-up facing the real Dr. Harvey as he freaks out, and it's a flipped shot—note his shirt and sweater are backwards (this is not a mirror's reflection)—right before he gets his foot jammed in the bucket. (00:31:05)
Other mistake: As the priest walks out of the haunted mansion with his head twisted, you can see the head and body don't step in time together.
Other mistake: When Kat chooses her bedroom in the mansion the bed should be covered in dust but it's not.
Answer: The Ghostly Trio were, at the time, playfully teasing James, so Fatso wearing something else instead of what Amelia was seen wearing in the photo, was just a way for them to have some fun at James' expense. At the end of the movie however, they did keep their promise and let James have one final chance to speak to his wife.
Why that specific outfit, though?
Again, to playfully tease James.