I had the same thing happen on an F2A that I owned in 2010. Happened twice, after trying out a 7.5/5.6 fisheye-Nikkor with the mirror locked up. After that episode, it worked fine. I've also had a couple junker Nikkormat FT series bodies do that. Except, with those bodies, you'd depress the shutter release, hear a slight click, then nothing else. Depress the DOF button, and the camera fires.