Stupidity: When Flint Marko is escaping from the police, he climbs a chain link fence into a test facility. Besides having a chain link fence, there should have also been security cameras around the area as well as guards to prevent anyone from trying to get in. Plus, when the scientist notice on their computers that there's an increase in mass, one of them simply assumes it's a bird and then they continue their experiment. Instead of making assumptions, they should have halted their experiment and have someone take a look to see what caused the increase of mass.
Stupidity: After Harry threatens Mary Jane that he would kill Peter if she doesn't break up with him, she does what he told her to. Why didn't Mary Jane warn Peter about Harry? Did she forget that he is Spider-Man? Even though Harry was watching them, she still could have warned Peter.