Revealing mistake: Casper is shown tying together the shoelaces of the students in Kat's class. When the bell rings and class ends they all get up and fall down. If you look closely you can see that their shoelaces are not tied together.
Revealing mistake: When Kat goes through the Up and At 'Em Machine, the top comb makes contact with her head, and a few of the comb teeth slightly bend. Thereby revealing that the teeth on the comb are made of rubber.
Continuity mistake: In the will reading scene after the lawyer has finished, Dibbs' hands are folded on the table, then off it in the next shot.
Continuity mistake: When going through the Up and At 'Em Machine, Kat is splattered with toothpaste. It remains on her face until the blow-dryer segment, and it's suddenly gone.
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.