Not a long time ago, I have started a project with urban night shots with my Mamiya 7ii.
I set the camera on "B" mode and I use also a cable release. When I use exposure times longer than 1,5 - 2,5 minutes, almost every time the shutter unlocks suddenly and accidentally, while the button is still pressed...
However, I haven't used yet exposures longer than 4-5 minutes.
I think it's not a matter of the specific cable as I had the same problem in the past with another one. Moreover, the problem also occurs if I use the camera button.
Any idea? Anybody with the same problem?