Ford Mondeo's in the UK had a mass recall when they were first released. A well placed high speed kick to the center of the front bumper would fire the airbag and unlock the doors even whilst the car was stationary and parked with no ignition on.
Though you'd have to really really kick it quite hard (excess of 15mph I think).
Lead to a spate of thefts and I believe it was reported on Watchdog (BBC TV consumer affairs program) years and years ago, at least I think thats where I saw it to.
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I worked for a while on a circuit that would control airbag firing - this was many years ago, but the issue that they had was a series of accidents where a car would jump a curb before hitting a building, the impact of the curb would fire the airbag, which would then have deflated when the impact of the building took place. The concept of our circuit was to wait to fire the airbag until there was an indication of metal deformation in the car.
Short answer - sometimes, on the early airbag systems, airbags popped with a relatively minor impact.
Seems to me the airbag wouldn't go off if the car wheels were not moving. Heck, I think they don't go off until the velocity is over some lower bounds. Don't need airbags going off at low speed fender benders such as happen in parking lots. The computer knows how fast the car is going.
There can be malfunctions. The anti-lock brakes on my Chevy truck have an issue with detecting speed. Part of the front wheel mechanism is marked and an optical sensor counts the marks as they go by. The idea is that the anti-lock brakes won't go off if the speed is less than 5 mph because at that speed, you are better off skidding. The wheel corrodes and the optical sensor misreads the speed. The anti-lock brakes kick in at low speeds and people crash into their garage walls at 2 mph because the anti-lock brakes won't let them stop completely.
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If the airbag doesn't go off when the wheels aren't moving, then when somebody drives into the stationary traffic at 40 mph, you're hosed. Exactly that happened to both my cousin, in Maryland, and a lecturer I had at college. In the lecturers case, it was a truck that hit his car as he waited at lights. Fortunately for him he drove a Volvo!
Anyhow, I'm sure they work when stationary, and if not, I'd have something to say about it. I think that's fake though.