Continuity mistake: After Kat finds a bedroom for herself, the long sheer white curtain hanging near the framed photo of Amelia repeatedly changes between pulled open or closed; particularly noticeable when Kat dumps her clothes from the black trash bag and tosses a ball of socks into Casper's mouth. (00:25:15)
Deliberate mistake: After James and Kat enter Whipstaff and get the power turned on, when Kat heads to the staircase to go search for a bedroom, we can see a few of the metal stair runner rods attached to the stairs to keep the carpet runner in place. However, when James falls at the top of that staircase and gets rolled up in the carpet runner like a sushi, there are zero stair runner rods. (00:21:30 - 00:31:50)
Continuity mistake: After Amelia grants Casper's wish to be alive (albeit for only a short time), when it cuts to Kat sitting alone at the party, note the distinctive worn chairs beside Kat. In the following shots, those chairs not only move their positions, they also switch places with each other more than once. (01:27:50)
Continuity mistake: Carrigan decides to make Dibs a ghost, so she goes after him with a double-headed axe. Then, when Carrigan slips on the muddy goo, her black pointy toe high heels turn into beige flat rubber-soled shoes, seen in her close-up as her foot lifts up. (01:11:45)
Deliberate mistake: After Stinky delivers his "Smell-o-gram" to James, he trips over the vacuum and falls, then continues down the stairway, with his body rolled up in the stair runner as it gains momentum. The problem is when the sushi runner hits the ground floor, the runner's edge is facing the opposite way to what it should be; deliberately done for the momentum and to have it humorously unroll. It's entirely impossible for it to turn itself around and then still have even more momentum to unroll the way it does. (00:31:45)
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.