Off the top of my head, The Lost Boys, From Dusk 'Til Dawn and the 2004 remake of Dawn of the Dead are offenders. The gripe? Once you turn into a monster, the human body apparently turns into pudding. Harvey Keitel points it out in FDTD, in fact, explaining that the vampires are squishy. The other two, in my opinion, much better movies don't bother to explain any rationale for it, we simply are expected to think that if you have something vaguely pointy and shove it at another human (or former human) that it will pass straight through them like a hot fork into Jell-O.
In case you are wondering what I am referring to; in the Lost Boys, when the Frog brothers and Corey Haim sneak into the vampire lair, a skinny 13 year old manages to put a thick, kind of blunt wooden stake right through Bill S. Preston, Esquire's sternum with about as much effort as sticking a knife into a hamburger. I m not suggesting yu try that exact thing, but just as a test, find the exact center of your chest. It's pretty hard isn't it? Now try to push your finger through it. Go on, I'll wait.
Still here? Good. If not, well, then I am wrong , but you are dead and can't complain. If you are still alive, though, that is because your sternum (the bone right over your heart that keeps your ribs from flailing like a Half-Life zombie's) is thick and hard, specifically to protect you very-important heart from being poked and therefore killing you. This is very important to would-be vampire hunters, since if we assume that vampires are real (they aren't, take off the black makeup...) and can live for centuries, we can assume that A. they are NOT made of pudding and would laugh off your retarded attempt to poke them to death with a pointy stick (seriously, no monster could be considered scary if this was a valid way to kill one) or B. they are made of undead tapioca, but would likely draw on their hundreds of years of not being dead to maybe put on some armor, or at least a thick sweater.
Corey Feldman's anti-vampire weapons are rendered useless by a douchey piece of clothing.
For zombies, it is their head. In the new Dawn of the Dead, the husband from Medium stabs a zombie in the chin with a broken croquette mallet and it comes out the top of his head. Holy SHIT. Be careful if you brush your teeth too hard, you could wind up with a splitting headache. (I am building a laser in my basement; once the book is published I am starting a new career as a Bond villain) The original had a zombie meeting his maker by getting a flathead screwdriver slowly ground into his ear.
I guess once you die, your skull turns into an eggshell. Except they can be repeatedly beaten with little or no ill effects; it's just once the stabby things show up, monsters turn into warm butter. I realize this is an easy way to make them not-too-daunting, or to get the character out of a corner they are painted into, but come on folks. At least they didn't do anything silly like give a nurse a shotgun.. Oh Wait...