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.
Continuity mistake: While Dr Harvey is fighting the trio his pants are cut off, seconds later he comes out of the closet and his pants are fine.
Continuity mistake: After Kat goes through the Up and at 'Em machine while sitting in a chair, we see Carrigan and Dibbs go through it, and they're in another chair. They then leave the chair and walk through the tunnel into the lab. Later, when Casper pushes Kat backwards through the machine, the chair that Carrigan and Dibbs were on is nowhere to be found.
Other mistake: When Kat chooses her bedroom in the mansion the bed should be covered in dust but it's not.
Continuity mistake: Before Carrigan and Dibs get in the mansion for first time he's soaked by the rain but seconds later he is dry.
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.