Stupidity: When Lois, possessed by Dawn, comes downstairs and says she's going to the prom and Clark is taking her, Clark doesn't think for a moment it could be Dawn. He already knows Dawn can posses others, those people don't act like themselves, Dawn is obsessed with the prom, Lois isn't acting like herself, and Lois is now not only suddenly prom obsessed for no reason, but wants to go with Clark when she said that would never happen.
Day 4: 11:00 P.M.-12:00 A.M. - S4-E17
Stupidity: When the couple is in the plant they call Jack on the cellphone, he instructs them to open the case and separate the book from the activation device. This was stupid, he should have told them to destroy it, since the codes had changed the moment Air Force One crashed, and it would take about 1 hour to make this work. This is explained later on in the show.