Stupidity: If the song "Beyond the Sea" can be continuously broadcast on the radio station as a "cryptic clue" that there are survivors living on the island and the aliens are unable to swim, then someone could verbally announce that information over the radio and otherwise communicate with other humans. The same thing about how Regan's hearing aid incapacitates the creatures. Rather than explaining how to weaponized the high-frequency sound, only the hearing aid's noise is broadcast and anyone hearing it has to figure it out.
Stupidity: Bori Khan is a skilled archer and a man without honor. He's not fighting fair, and cares just to win, as established and explicitly said. He shoots an arrow at Mulan, the Witch gets in the arrow's way sacrificing herself to protect her...and for no reason whatsoever he does not shoot anymore, giving the chance for the two girls to share their very special dramatic moment together, but creating a colossal plot contrivance. It is a know movie cliche for the fight to 'pause' around the main character, but here we have a sniper who desists completely (he won't shoot anymore) for absolutely no reason. (01:29:00)
Stupidity: At the outdoor market, Libby told her old girlfriends that she'd "be right back" and went into a stall in the women's room. Soon, her girlfriends came in but stayed in front by the mirror. Without bothering to look if anyone (particularly, Libby) was in one of the three stalls, they started talking negatively about Libby (e.g, " Ugh, what is it about her that is so irritating?", "I feel sorry for her", "She looks like she's wearing a Halloween costume", "She's like a lumberjack"). (00:55:08 - 00:56:00)
Stupidity: Becky possessed the mysterious key that the four neo-Nazi prison escapees were at the lake house to retrieve. Becky didn't know what the key was for, so did not have a vested interest in it. By first denying that she knew where the key was and later refusing to give it to Dominick, her father was killed. Had Becky simply handed over the key when Dominick asked for it, it is possible no-one would have been hurt/ killed. Becky keeping the key with no importance to her served to further the plot.
Stupidity: Shortly after leaving the man in the hole, Joey met Matt at the top of the embankment and told him he lost the gas mask. Instead of backtracking (not very far) and trying to find the mask (fingerprints on it could be evidence against him), Matt tells Joey that he owes him "30 bucks."
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.