Continuity mistake: When the kids light the two lanterns they find in the pipe room, they stay lit somehow, even though the lanterns get wet in the well.
Continuity mistake: Before the 'bathroom' break, when Mikey holds up the map, the top of it is burned off and very few musical notes are actually there along its edge, but when they reach the skeleton's bone keys the top of the map is clean, with a musical score along the entire edge. (01:29:30)
Continuity mistake: Chunk breaks the glass on the frame and then he and Mikey clear away much of the broken glass from it. When there is another view of the picture all the glass is still on it.
Continuity mistake: When Andy begins to lose it in the tunnel, Brand holds one lit lantern at the head of the line and Mouth holds another lit lantern at the back of the line (Mikey holds an unlit lantern). Next shot the lit lanterns have switched, just as they reach Chester Copperpot and Andy screams. Then in the next wideshot before it pans to the skeleton, the lantern Brand holds up switches yet again. (00:53:15)
Continuity mistake: When Mr. Perkins hands Brand the foreclosure papers and then when the two men walk away, the picket fence door is shut, but when Mikey is alone on the porch as he looks at the two men, that fence door is wide open. (00:21:10)
Continuity mistake: In the opening scene where Data is using his sticky dart on his belt and being dragged towards the full bin, it changes to being empty just before being pulled in! (00:03:50)
Continuity mistake: After letting the air out of Brand's bike tires, Mouth shuts the picket fence door (the door near the boat) and then Mikey begins to ride away on his bike, but next shot as he rides past, that fence door is wide open. (There are two fence doors - one near the boat and the other one Chunk used earlier.) (00:23:25)
Continuity mistake: At the start, when Data shoots the suction dart at the orange drum, the drum's scratches, dirt and dents are different in the following shot, as it is being pulled towards Data. (00:03:55)
Continuity mistake: When Chunk drops the frame with the map, in the shot looking down at the frame lying on the floor, there are five narrow planks of wood flooring under the frame, but next shot there are three wide planks. (00:16:10)
Continuity mistake: After Brand is given the foreclosure paperwork, when everyone goes inside except Mikey all the hanging plants at the side of the house are gone, but they're back when the kids leave on their bikes. (00:21:05 - 00:23:45)
Continuity mistake: After Chunk walks in, when Mouth and Mikey kneel in front of the aquarium Mouth's white sock is visible, but when Chunk places the statue on the edge of the table and it falls, we see that Mouth's sock has vanished. (00:08:10)
Continuity mistake: When Data explains that the skeleton is Chester Copperpot, Mikey is crouching beside the boulder that fell on Copperpot. In shots facing Mikey, he's holding the book and baseball card, but in shots facing Brand, Data and Mouth he is not. (00:54:25)
Continuity mistake: When Mr. Walsh has the paper on Mr. Perkins back, the top paper reads "final notice" when Mouth turns to stop him from signing, but when it cuts to the close-up, that "final notice" top sheet is actually gone. (01:46:10)
Continuity mistake: When Mrs. Walsh asks, "What is that?" pointing to the mess on the table, the bowl is filled with chips in the close-up, but when she walks out with Rosalita the bowl is nearly empty. (00:13:00)
Continuity mistake: When Data places the cassette into the tapedeck and grabs hold of the bar on the zip-line, his palms face him as he takes off, but next shot as he slides down his palms face out. (00:08:50)
Continuity mistake: After Mr. Walsh tears up the paperwork, when the reporter steps forward to ask if the jewels are real, Mouth stands right in front of him facing the other kids, but next shot Mouth instantly stands behind the kids, facing the reporter. This continues to go back and forth. (01:47:00)
Continuity mistake: When Brand, Andy and the rest enter the room with the fireplace and printing press, the water cooler is covered in cobwebs, but when Chunk knocks over the water bottle just about all the cobwebs are gone. (00:40:25)
Continuity mistake: When Data shoots the blue suction dart at Mama Fratelli's gun, it leaves the stick behind still attached to his belt, but in the close-up of the gun the stick is attached to the suction cup and string. (01:31:50)
Continuity mistake: At the start, in the second exterior shot of the Walsh's house, after the Fratellis drive past at the bottom of the hill, we see Mikey sitting outside his window beside the small, all black telescope. In the next interior shot the telescope is a different style, it is now grey/black and quite larger. (00:05:10)
Continuity mistake: When Mikey opens Chester Copperpot's wallet, the wallet and what's inside differs between shots, as well as the way Mikey holds it. (00:54:55)
Answer: According to Mikey and Mr. Walsh, one of his crewmen must have taken the map and fled when One Eyed Willie was murdering his crew. This is substantiated in the novelization.