I had this exact problem in 2003. After puzzling and getting a bit of input from Photo.net, I managed to work it out and brief a rather reluctant Bronica repair facility accordingly. Basically its a MLU problem, and here's my final posy on the issue which I hope will help.
"In case this happens to anyone else I should report back. Ken Heflinger was closest to the answer.
It’s important to understand how the camera works normally. Basically the camera sits with the shutter wide open so you can compose and meter TTL. The film is protected from fogging by the mirror which forms a light-tight seal with the body. So , if you lock up the mirror, the film would fog unless the act of locking up the mirror closes the shutter. So that’s exactly what happens. The shutter closes as the MLU takes effect to avoid exposing the film.
The fault mode here is exactly this. Once in a while, for reasons and triggers not known, the MLU on this body doesn’t work properly. Instead of going all the way up it stops part way. Importantly, when this happens the shutter doesn’t close. So the mirror’s part way up and light is streaming through an open shutter, past the half-up mirror, to fog the film. The light strikes mainly the bottom half of the piece of film you’re about to expose but the effect of fogging extends to the top half of that frame and the top half of the frame following -so equidistant up and down. As soon as you take (or think you take) the affected frame you wind on and the mirror comes back down. However the damage has been done before you take, and not only do you fail to make an image on frame 1 but you’ve also got a half-fogged frame 2 behind a now functional mirror.
So how do you get round it till you get a repair? Well your only chance, outside of ceasing to use MLU, is to hear the difference when you turn the switch to lock up the mirror. It’s pretty distinctive but you’ll miss it if preoccupied. When you hear the malfunction, expose two frames, and start the process again with the third. The necessary repair is to the MLU switch/activation. Don’t let them mess around with your lenses or the shutter control mechanisms in the camera. It’s the MLU, pure and simple.
So how do I know that it isn't exactly as Ken suggested- the mirror failing to return fully on winding on after an exposure. Well, only because that would leave me with a black viewfinder for when I'd be trying to compose my next shot- which would be a kind of giveaway that something was wrong! I had no such clues
