In the classroom, Casper ties Amber's shoelaces together before anyone else's. When Amber stands up to protest the party being at Kat's house instead of hers, she doesn't trip over them. She sits back down, but when the bell rings and she gets up, she trips.
When Carrigan is walking down the hall with the axe in her hands looking for Dibbs, the bottle of potion is nowhere to be seen. When she goes through the window, the bottle is lying next to her on the ground.
When Casper is pouring the fresh orange juice for Kat, he collects the pulp in his hand. When he tips the pitcher up, the orange juice pouring through his hand stops just before he tips the pitcher back.
When Carrigan and Dibs flee from Whipstaff during the storm, what appear to be the long sleeves of a red jacket are clearly visible draped over a large concrete flower pot to the right of the front door. Carrigan and Dibs drive away, and right after their SUV exits the frame, we see that the red jacket is gone. We can also see that this was a single shot without a cut, meaning that somebody had to be on the porch, out of sight, and able to remove the jacket in the two seconds it took the SUV to pass in front of the flower pot.
When Dr. Harvey rolls down the stairs, he gets wrapped in the carpet on the stairs. When he gets to the bottom of the stairs he keeps rolling, and the carpet unrolls itself onto the floor. He going in the wrong direction for that to happen.
When Casper grabs Kat's arm and drags her toward the window out of the room, she leans back and gets dragged on her heels, thus pulling her arms stiff straight out in front of her. Before they're out of the room, he starts to lift her in the air but Casper is still in front of her and holding her arms stretched out in front of her. He should have been above her head and her arms stretched out above her.
When Kat is dancing with Casper at the end of the film, the duo is shown floating above the floor. Yet, in the next shot, a view from the balcony, they are back on the floor.
However, during a shot in between while he's identifying himself to her a light in the background can be seen getting bigger indicating that he's lowering them to the ground.
Casper tells Kat that after death, life doesn't matter so much anymore, so things tend to be forgotten. Every time a person becomes a ghost, they forget who they used to be and what their purpose was. When Dr. Harvey dies, he can't remember his own daughter. Yet, after Carrigan dies, she still remembers her partner and her duty (to get inside the vault). The ghosts in the movie regain their memory quickly. Carrigan may have remembered her role as soon as she saw Dibs.
Ghosts don't have a reflection. When Casper looks into a mirror, he can't see himself. But when Amber and Vic are dressed in costume for the party, Stretch, Fatso, and Stinkie spook them, and appear in the mirror. However, these are not their reflections. When the two kids fall, the ghosts are nowhere around, suggesting that the Ghostly Trio is actually inside the mirror.
It is clear that the ignition must be turned upwards to remove the resurrection liquid, as shown by Dibs when he stole it. But when Carrigan is turned into a ghost and he refuses to make her live again, he pushes the liquid out of the ignition with it turned downwards. That gesture should have caused the opening of the bottle or the destruction of the ignition. The bottle does not open, but the ignition sparks. That shows it is broken. How they brought Dr. Harvey back to life if the ignition is broken?
Casper states the 'resurrection liquid' that is in the bottle is 'enough for one [ghost]'. When Casper and Kat bring Doctor Harvey back to life, there's still liquid. That could make Casper come back to life, but they don't use it.