Stupidity: Dr. Jekyll must take an injection to keep himself from turning into the murderous Mr. Hyde. Hyde obviously doesn't want Jekyll to take the injections because he wants to remain in control of the body. The time between when he begins to feel himself turn into Hyde and when he has fully transformed is ridiculously brief. Why then does he keep the syringe locked in a box in his desk rather than on his person at all times?
Stupidity: Once the casket was left in the mercury in the tomb, the ropes and pulleys should have been taken away instead of left in place for it to be retrieved. For something so evil why would any sane person want to bring it back out?
Stupidity: When Henry is talking to Nick, he feels himself beginning to change into Edward Hyde, he goes to his desk to inject the serum to stop his transformation. If he had injected himself before continuing the conversation, his transformation into Hyde would not have occurred.
Suggested correction: The first time Henry felt himself about to change into Edward Hyde, he quickly took the injection so it's most likely he only takes it when he first feels the beginning of his transformation.
