A one stop error in overexposure should have produced an image, although the highlights would have been washed out. The fact that you got a much more overexposed image (according to your description) would mean something else was not quite right. So, you did remember to stop the lens down after focusing? I have had (on an older Ilex shutter) the little screw come out that attaches the external ring and pointer to the internal iris and thus you could turn the outside pointer all you wanted but the iris wouldn't stop down. Check to make sure your iris stops down when you adjust the f-stop pointer on your lens. Also, if you left your shutter on "open"-for focusing then you could have gotten overexposed images, even though you thought it was giving you a 1/50 sec.