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.
Plot hole: When Harry's pumpkin bomb exploded, disfiguring his face, there were also more pumpkin bombs around him. This explosion should have caused them all to explode, which would have killed Harry.
Suggested correction: It's entirely dependent on how the pumpkin bombs function and how they're built. They're likely built to be able to withstand a certain degree of outside force and heat so they wouldn't just randomly explode during a superhero/villain battle. They also don't seem to function like regular grenades... there seems to be electronic components to them. So it's entirely possible they can't go off until they're activated because certain components inside haven't mixed or been armed. Their rules are just too vaguely defined for this to be a valid mistake.
Suggested correction: Mary Jane didn't forget that Peter was Spider-Man. A great deal of time passed between when Harry talked to Mary Jane and when she was forced to break up with Peter. During that time, Harry could have threatened to kill her family if she didn't cooperate with him.
Mary Jane doesn't take into consideration whether Harry would have the capabilities to kill Peter since Peter is Spider-Man.