Stupidity: Captain Poison has a fairly good idea of where Solomon buried the diamond. We know this since when he leads the captured Solomon to the spot, there are already several holes dug looking for it. Despite knowing roughly where the diamond is - down to a relatively small area - he makes no serious effort to excavate the ground, instead contenting himself with a handful of incredibly shallow holes.
Stupidity: After John's wife is taken he follows them. Four of them spend a good 5-10 minutes shooting the hood and down the side, he pulls right up next to them and they still shoot down the side. Why didn't they just shoot right inside the windows or or windshield? Should have been very easy since all the windows were down and John had knocked out the windshield. John would have been dead and their troubles over.
Stupidity: There is a scene in which the phone rings back at the house and Andy goes to answer it. None of the henchman respond immediately, but instead WAIT for him to almost answer the phone before becoming making a move. You would think they would yell at him to not answer it or have one of them rush to guard the phone the moment it rings. Obviously the entire scene was done this way for the sole purpose of suspense.
Stupidity: If the weak spot of the house is the fiery chimney, then the kids could have just thrown the dynamite into it instead of lighting it themselves. They put themselves in unnecessary danger for the sake of drama.