Pierangelo,
Did you use times of 1 second, or thereabouts, on only that one shot?
If you do not keep the release depressed for the entire duration of the exposure, the rear auxillary shutter will slam shut while the lens shutter is still doing its thing, resulting in what could be massive underexposure.
So there's one suspect.
You can see whether your magazine is an "A" type by the presence or lack of a foldable wind crank (as opposed to only a key) on the counter side.
I asked, because in older non-A type magazines, the dark slide operated shutter blocking feature would be lifted as soon as the darkslide was pulled out only a tiny bit. You could then take pictures and only expose the dark slide instead of film, creating beautifully unexposed frames.
Taras' suggestion about the magazine being removed and reattached to a camera in released state is also a good one. It can happen quite easily.
No use of flash rules out the bad PC-contact theory. ;-)
It's always possible that the shutter has an intermittent fault (rather a constant fault showing itself not constantly), not opening now and again. You can check if it has in a very simple way. Take of the magazine, and start firing, the full range of shutterspeed settings, many times. 3 out of 24 is 12.5%, so if there is a fault you should be able to see it within a reasonable amount of test firings.
Either look at the shutter through the front of the lens, or (better) through the back of the camera. You should see the thing open, even at the fastests shutterspeeds.
(And if you are not familiar with the "finger-timed" rear shutter action, see (and hear) what happens when you let go of the release button immediately while firing at 1 sec.)
