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.
Stupidity: Spoiler: Everything goes well in the end and all the ghosts (and returning demigod Gozer) are sucked into a giant ghost trap. In fact everything goes so well that the trap selectively spares one specific ghost so it can have a lengthy farewell scene, hug and smooch every single character at leisure and leave on its own terms.