Other mistake: In the final scene where the kids are being asked what was the scariest part of the trip, Data replies "The octopus was scary." But the scene containing the octopus was deleted from the movie. The scene is shown on the DVD's deleted scenes. (01:47:00)
Other mistake: When Troy is in the country club restroom and is sitting on the toilet, he has his trousers down around his ankles. When he gets shot up and crashes into the ceiling, you will notice he has red briefs on, but surely he would have pulled them down too... (00:51:20)
Other mistake: With Andy weighing at least 100 lbs, surely Troy and the guys trying to pull "her" up would notice that a bucket carrying the sweater is nowhere near the weight of a person. Even if they filled it with coins from the wishing well, it still wouldn't come close to the weight of an actual person, regardless of how idiotic and pompous Troy and his two friends are.
Other mistake: When Andy plays the bone organ, the musical score on the back of the map clearly shows a melody consisting of single notes with only one or two chords. Still, Andy presses multiple keys on the organ to create chords EVERY time, not some single notes as the code shows. The code should not have worked if a person was playing only chords.
Other mistake: When the Fratellis get out of the water, they show no signs of being wet at all, like when they catch the Goonies near One Eyed Willy.
Other mistake: Up in the Walsh's attic, in three shots of Mikey as he continues to say, "Then he loaded it all up onto his ship and he sailed away into the sunset..." there is something black hanging down from the top of the screen that stays put as the camera pans in closer to Mikey's face. It disappears, then reappears when he says, "Trapped." (00:17:20)
Other mistake: In the pirate ship scene, when the large boulder is falling from the ceiling of the cave into the water, its splash is far too small.
Other mistake: At the end, Mouth goes up to Stef and thanks her for offering to save his life. During the movie, however, Stef never offers to save his life. It is explained somewhere that in one of the deleted scenes, when Stef and Mouth are on the plank she offers to share her breath with him so they can both breathe under water.
Other mistake: After the kids find Chester Copperpot's skeleton, Mikey picks up Copperpot's book and finds the Gehrig baseball card inside. When Mikey turns the card over it's the 1973 Topps Lou Gehrig All-Time Grand Slam Leader #472 (which has nine players' grand slam stats listed on the back of the card, that includes Hank Aaron who started playing MLB in 1954). The problem is Copperpot was reported as missing in 1935, but this specific Topps baseball card (now well known) was issued in 1973, nearly 40 years after Copperpot's death via the pirate booby trap. (00:54:30)
Other mistake: In the scene where Mikey, Brand and Mouth are listing the exaggerations Chunk has come up with, Mouth says "And I bet it was even more amazing than the time you ate your weight in Godfather's Pizza." When he gets to the "Godfather's Pizza" bit, it becomes clear that this has been dubbed over the original filmed scene, as his mouth does not match his words in the slightest.
Other mistake: In the shot of the old guys taking a shower, you can see the outlines of the tiles that are supposed to be smashed when the water regulators withdraw into the wall. (00:50:50)
Other mistake: When Data puts his tape in the stereo he shuts the thing then the 007 music plays. He doesn't ever push play.
Other mistake: In the Summer Place scene, after Mama Fratelli sets down four glasses of "colored" water for the boys to drink. The next camera angle shows Mouth wiggling his glass in the air as he retorts, "We're supposed to drink this?", Mama snaps back, "It's wet isn't it? Drink it." If you look closely for 2 seconds you can see Corey Feldman is trying very hard to not smirk at this line, while struggling to stay in-character, as Mikey and Data say their next lines.