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These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Instead of either telling everybody about the big tentacle monster and showing them the cut-off part or keeping their mouths shut completely, David and the other guys do everything they can to come off as babbling idiots to just about everybody and blow their only piece of evidence on just a couple of people.
When the front window starts getting flocked by giant bugs, nobody spends even two seconds to think "gee, maybe they are being attracted to the lights just like normal bugs tend to be". Instead, the crowd fills the front window with lights, some idiot turns on the floodlights in the back and it is only after the window starts getting smashed up by the bat creatures that somebody thinks that doing this might be a bad idea.
On the trip to the pharmacy, the group takes its sweet time browsing and grabbing an entire pile of pills, when they're all on their toes and should be working as fast as possible. And when one of them starts hearing strange noises, do they get the hell outta Dodge? No, they investigate the noises, and when the creepy crawlies inevitably show up, instead of legging it like they should, they just stand there and scream. No wonder they lost two people.
Despite being desperate for communications throughout the entire film, nobody thinks to look in the MP's jeep, which is parked only feet from the front door. When Laurie Holden produces her gun, there's also a discussion about who is qualified to carry it - ignoring the three soldiers in the store. Even if the townsfolk didn't trust them, they aren't even mentioned. Similarly, despite characters voicing suspicions that the military was somehow involved (and David observing them behaving oddly before arriving at the store), nobody thinks to ask the Soldiers if they knew anything until days into the disaster, and its too late as the only two who knew anything had already killed themselves.
Oh, and the big one: Letting Mrs. Carmody preach to the crowd. Her speeches do nothing but stress the survivors out even more than they already are and one of the characters notes early on that she might cause real problems if she manages to gain any followers. Yet it takes most of the film and her calling for the sacrifice of David's son to make somebody finally take care of her. Yes, the central conflict of the movie could've been easily resolved in 10 minutes through a liberal use of clothesline, duct tape and a bathroom stall. Nice job, protagonists.
Moral Event Horizon: Mrs. Carmody starts off unbalanced (though more conflicted and likeable in the movie than the original story), then becomes the chosen messiah of the people, and starts ordering human sacrifices to the monsters outside. Okay. This is a horror movie. Then she orders David's son and the woman who had been keeping the kid calm this entire time to be thrown to the monsters...
And by convincing her cult to sacrifice Wayne Jessop, she probably attracted even more monsters to the store by putting the smell of blood and an easy meal into the air.