Didn't Romero slice off the top of a zombie's head with a helicopter blade in Dawn of the Dead?

Yes, one zombie got the top of his head sliced off in the original DotD. If I remember correctly, the helicopter was sitting on the ground with its blades rotating.

There was a similar trick in the James Bond movie Tomorrow Never Dies, where Pierce Brosnan and Michelle Yeoh are chased down a street in Bangkok by a helicopter with its blades angled down toward the street.

Any helicopter pilot will tell you, though, that such a maneuver is aerodynamically impossible. A helicopter accelerates forward by angling its blades and tipping its nose forward... so if you had that much angle, you'd really be accelerating quickly forward (not hovering in place or even moving slowly), and you'd be dropping (since a helicopter sacrifices lift for thrust when it accelerates). So, the helicopters in Grindhouse and 28 Weeks Later should both have crashed into the ground. Also, any impact of that scale on the rotor blades would cause severe damage to the blades and supporting struts, and the helicopter would probably crash fairly quickly even if it was just hovering. (My dad was a pilot and he used to explain stuff like this to me all the time. His favorite movie for bad aerial behavior is Top Gun.)

I know, it's just a movie... but it's hard to suspend disbelief sometimes. ;-)

Good point.

Of course, in 28 Weeks Later they're not really zombies. ;-)

Well, that's the premise upon which the whole movie is based, so as long as it operates with a consistently-applied set of rules (e.g., how the Rage spreads, that there's no cure for it, etc.), it's cool.

For example, a scripted-but-thankfully-not-shot ending for the first movie involved them "curing" someone by draining all of the blood out of their body and replacing it with new, uninfected blood; the writers wisely abandoned this idea after realizing that they'd already established that even a single particle of Rage virus would infect you and that there was no way to remove all of the Rage from an infected person. Make your rules, and stick to them. The idea of an immune carrier doesn't violate previously established rules for the Rage virus, so no problem there.

Beyond that, my suspension of disbelief tends to break down when movies have stuff happening which just doesn't make sense in the world we live in. Assault rifles firing hundreds of rounds without reloading. Explosive decompression when someone goes out of an airlock. Car gas tanks that explode when shot with bullets (Mythbusters recently debunked this one). People outrunning fireballs from huge explosions (The Long Kiss Goodnight, among others). Spaceships that fly like airplanes (Armageddon). That sort of thing.

I realize these things are done for dramatic effect, but it is possible for a story to be really good and still make sense... the Bourne movies, for example, have really good stories and action and yet Jason Bourne is clearly no invulnerable überagent like the old James Bond... when he takes a big jump, he twists his ankle and limps for the rest of the movie.

I just think it would have been more satisfying for me personally if they'd left the rotor-blade-slicing-up-the-Infected bit out of this movie. The other stuff I've mentioned can be chalked up to human error... this sort of thing has cropped up before, and it has always been due to human error. ;-)