When Chunk breaks the glass frame of the picture up in the attic, Mikey clears the debris off to look at the map. But in the next shot, the glass is all still there and they clear it off again.
I noticed this too and I have a question. Mikey 'set it up' for Chunk to break the glass....right....right. However, when Mikey is clearing off the glass to look at what is underneath, he asks 'What are you doing?' Who is he talking to? Himself? He is the one with the suspicion and he is the one clearing the glass. My opinion is that was supposed to be someone else's line and no one notice!
I think it's one of two things: Either Mikey just didn't want the others to know that he set Chunk up to knock it over (unlikely, IMHO), or...originally in the script, maybe Mikey wasn't supposed to be so "gung ho" about the idea. Maybe he got mad because Chunk broke something, but then decided that a pirate treasure would be pretty cool. So they filmed that scene one way, then decided later that it would work better if Mikey wanted to break the glass so they could get to the map.
I have watched this movie an infinite number of times, and there is no mistake when Mikey says "what are you doing?" He says this when Chunk picks up the frame and throws the broken glass off into the attic. He set Chunk up to brake the glass, because he didn't want to be responsible for breaking his dad's priceless map--but I would be upset too if someone threw broken glass all over my attic!