OK, silly to ponder something so easily tested: Indeed on an F-1N you can have the DOF preview lever actuated, mount a lens, then put it back to its normal position, and it won't jam the camera. (The aperture obviously stays wide open when you take pictures, regardless of where the ring is, but it doesn't jam the camera.)

Anyone with an older F-1/F-1n want to try this, just so I know I'm not going insane when I remember that as being a case where the camera would no longer wind until you removed the lens and got things put right again? I definitely remember thinking that red dot needed to be larger to warn you more! The classic scenario is unmounting a lens when you have the DOF preview lever engaged... you can't then mount another lens until you've unengaged it first.

Duncan