Stupidity: Throughout the movie, the Virtuoso did many things that were contrary to being a shadow and not leaving any evidence of his presence. For example, he often wore gloves, but not when picking up a knife in Rosie's kitchen. Each time he went into the motel office, he interacted with the clerk and called attention to himself by asking for coffee but leaving before the clerk returned. He fell for the Waitress' advances, met her after the restaurant closed, and let her come into his motel room.
Stupidity: Instead of breaking into Gordo's house, covering all the furniture in one room with sheets/thin blankets, and waiting for Gordo to return home in order to kill him via a "dramatic re-enactment" (in an adjacent room which was not shown to the audience), Bill could have taken Gordo anywhere to avoid getting caught or calling the police on himself. Had Bill taken Gordo to his already-"wrapped" motel room, he had enough injuries after the fight to claim self-defense against an intruder. (01:43:20)
Stupidity: To quietly kill the hotel valet he is replacing, Diabolik throws a knife at him from distance, hits him in the back, and then drags his body in the middle of the street to the nearest manhole, leaving a long streak of blood he does not conceal. If this sounds pretty risky and unsafe especially for a meticulously planned murder, you have also to consider that in the comic book from 1963 this movie otherwise faithfully adapts, Diabolik did nothing this foolish; he quietly killed the guy strangling him next to the spot he'd dump his body into.
Stupidity: Patrizia Reggiani is no rocket scientist in this movie, but it's quite bizarre that calling the fortune-teller she sees on TV, with her name written in giant letters in two different spots on screen, she asks her "What's your name?" It makes sense for the international audience who may not recognize the word "Pina" as a name, but an Italian wouldn't have any doubt. (00:41:05)
Stupidity: Lea had two "Notice of Auction" printouts (the correct one and the one she lied about to detectives) in her small purse when she had Casi meet her at a restaurant. Lea excused herself to go to the bathroom but did not take her purse with her. Women almost always take their purses with them and Lea should have, considering she had papers inside she didn't want Casi to see. While Lea was gone, Casi looked at the printouts and then knew Lea was lying to him about the Navigator's auction date. (00:32:33 - 00:35:04)
Stupidity: Jimmy parked his truck on the side of the road with woods running parallel and starting about 10' away. Jimmy moved to the passenger side so the Sheriff could sit in the driver's seat (for no apparent reason). After Jimmy threw the "white powder" into the Sheriff's face, he quickly got out of the truck and ran straight ahead on the road instead of ducking into the nearby woods. The Sheriff had a clear and easy shot to hit and kill Jimmy. Also, Jimmy should have been much further away. (00:11:51 - 00:12:21)
Stupidity: Why would the tow driver go around Deacon's truck in the one-way dead-end parking lot to pick it up from the front? He'd have to back it up on the hook, and Deacon's truck would be in Park, making it impossible. There's a reason tow trucks hook up vehicles by the drive axle ends. (00:08:40)