Stupidity: Friend Request: Michael sneaks into LifeInvader as an IT temp so that he can plant an explosive, which he is carrying in a blue bag, into the prototype mobile that Jay Norris is going to use. He leaves the blue bag, evidence tying him to the explosive, in the LifeInvader building.
Stupidity: The protagonist removed a blood-stained bottle from the bloodied car where the murder takes place, and this may incriminate him. To avoid being found out by the police, he disposes of it where, he unironically comments "nobody will find it." Which is...the closet of his bedroom. It works.
Stupidity: There's no explanation for the level in Italy; Billy and Jimmy have already all the three stones by the end of the Japan battle and they need then to go to Egypt. The NES version does explain it, in a laughable way; Hiruko says a rather silly "We must now travel to Egypt to free Marion, but before we do, we must spend time in Italy, practising for the big battle", Technically the NES version has a different storyline, since Marion is not missing at all in the other versions.
Stupidity: During the final phases of the Turnabout Sisters case, trying to press Redd about him not being solely guilty of wiretapping will cause Edgeworth to state that "unidentified fingerprints several days old were found in the Fey and Co. Law Offices" and they'd "obviously" belong to Redd. That's an unnecessarily ridiculous non sequitur; being an office with several clients, it'd be astonishing if there were no "unidentified prints" at all.
Stupidity: Despite having a gun which is shown in kill animations, Imposters can't kill crewmates from the other end of a hallway.
Stupidity: During the Valentine's day event, the bad guys attack "chocolate factories" the day of St. Valentine to spoil the festivity so "there will be no chocolates to buy." That's not a very timely plan; even if they wreck the factories that day, the chocolates have been already delivered to the stores.
Stupidity: In chapter 7, Reptile mentions that he will turn invisible and cause a distraction for the rest of the heroes to manage to escape from the Outworld military. Moments later, Reptile causes the distraction way too close to where the heroes were, which could very easily blow up the gang's cover and make them come face-to-face with the Outworld military that was after them or injure them by accident from the explosions caused, Plus Reptile isn't even invisible, which gets him spotted and recognised.
Stupidity: In chapter 2, when Luke arrives home he thinks "Maybe she went away?" Tanya says "No, I am in the bathroom." Luke then correctly asks if she can read his mind, and she says that she can't, she just reads the signals human send unintentionally. Which is fair, but she's in the bathroom and she can't see him at that point.