In the final scene, Elliott says "every killer leaves something behind." Stuckie rebuffs him, stating all he has to do is get rid of the mosquito, then he can walk away, free. This is not accurate- Dale is not wearing gloves when he stabs O'Halloran with the knife or when he picks up Olivia's gun. Even if he had killed both Elliott and Olivia, his fingerprints would be all over the weapons and the duct tape on Elliott's mouth.
When Harrison first opens his apartment door, he leaves the chain in place. The camera cuts back and forth between Harrison and the detectives a few times, then the chain is suddenly gone without Harrison having had enough time to remove it or making any noise indicating that he was/is taking it off.
After Olivia takes the duct tape off of Elliot's mouth and puts it on his shirt, the position of the tape on his shirt changes it between shots. In the shot of Elliot watching Benson kiss Stuckey, it disappears altogether, then reappears as he is kicking Stuckey.
One of the victims is attacked with a syringe of potassium chloride (the same basic ingredients of lethal injections). However, if this is injected into a person without being properly diluted as is done in executions, it will burn through veins and skin which wasn't seen or mentioned.