Stupidity: Jill wasn't thinking too clearly when she plans her story. She only wiped the fingerprints off the gun but they would be all over the tape on Trevor's body. Then she wipes off the knife yet touches it again to slide it across the floor. She was thinking that Trevor and Charlie were going to be thought of as the killers but their fingerprints weren't on either of the weapons.
Stupidity: As the movie progresses, we see the deranged killer, Martin, collecting a dozen victims for his Human Centipede project and depositing the victims in a sealed chamber. Every time we see the victims, they are nude, face-down (always in the same positions on the floor), struggling and moaning and sobbing. However, their hands are only duct-taped behind their backs, and their ankles are duct-taped. There is nothing to prevent these victims from rolling over, sitting up, standing, and even assisting each other to escape. Yet they never change positions or attempt to escape.
Stupidity: In the opening scene, Sonny has come to her now-deceased father's house. Police and the coroner are in the basement taking notes while standing around the body - which is face down and awkwardly lying at the bottom of the steps. Det. Sabatino informed Sonny that the death was ruled accidental due to a high blood-alcohol level. Detectives or the coroner did not rule out other possibilities: natural cause, suicide, or murder. High blood-alcohol level could be secondary.