Stupidity: For Sarah's murder to work, a lot of coincidences and unpredictable, convenient behaviour from the victim have to happen; she has to Miss the security guard and not hear them, she has to seek shelter of all places right in the costume room full of effigies of the killer, where the killer himself happens to be posing, she has to be completely inefficient at calling help with her cell phone.
Stupidity: Throughout the movie, and especially up on the top of the apartment building before they get rescued at the beginning of the movie, Gerry counts 12 seconds to check if someone has been infected. Just to mention this apartment example, he gets blood on his face and spends more than 12 seconds getting up and to the edge of the roof, plenty of time to become "Zeke." He then counts 12 seconds, as if they would actually matter to his infection.
Stupidity: The whole movie happens for unbelievably convoluted causes. Despite their friendship and the simple fact that he still runs an occult shop (showing that he's not exactly insensitive to the past), Ray in years never spent a single moment to check on Egon's whereabouts, which he knew, or listen to him. The city has Shandor's name all over and any cursory investigation would have unveiled the connection. Assuming of course that Egon turned into such a lunatic he couldn't do the namedropping himself.