Revealing mistake: On Inferno's deck, when Data takes a stand against the Fratellis before he shoots the blue suction dart/string, the close-up of his utility belt is the same one seen at the start when he practices on the orange drum. Note the differences in the background and clothing/inventions under the coat. (01:31:50)
Revealing mistake: When the bats escape from the fireplace, it's obvious they are just black pieces of material.
Revealing mistake: When Francis Fratelli is trying to pick up the gun as the Goonies are escaping from the organ, you can see the rocks shaking each time he drops the gun on the ground.
Revealing mistake: When Mikey is on the balcony he turns towards Bran to hug him, and then Bran carries Mikey all the way to back of the balcony. During that whole time there are leaves falling from the sky. All the trees are outside of their picket fence and lower than their balcony roof. So they must have had crew on the roof throwing leaves down for the effect. (00:21:10 - 00:21:40)