Continuity mistake: During the battle with Green Goblin, when Gwen Stacy is falling through the clock tower, there are dozens of gears and other pieces of various sizes falling with her. However, when she lands, only a few small gears and pieces land alongside her - all the other debris that were falling have seemingly vanished. (02:01:20)
Stupidity: Harry Osborn quite literally inherited the company his father founded. Presumably he owns or controls a majority of the stocks, because he was appointed CEO by his father and nobody questioned him. However, one of his employees can just instantaneously fire him from his position. We don't know the precise rules and internal regulations of Oscorp, but it's safe to say that this is not how company hierarchy works, especially considered that no charges are pressed on Harry and everyone would be out of a job (including Menken) if the circumstances were public - like having created a monster and waterboarding a guy in their basement.
Continuity mistake: Harry and Electro are in the office and confront the new CEO. Electro shocks him, and he falls to the floor. The first shot of him is clearly not the same actor, because the man on the floor has a full head of hair. The second shot it's the original CEO, who is mostly bald. (01:40:55)