This must be a Nikon thing or I have been screwing up for a long time because I still don't get what your talking about.
Nikon's full-frame DSLRs have the ability to shoot in "DX Mode", meaning only a portion of the sensor is used to create the image.
They can be set three ways:
FX Mode, which will use the entire sensor regardless of whether there's a DX lens attached or not,
DX Mode, which will use the center portion of the sensor regardless of whether there's a DX lens attached or not,
DX Auto Mode, which will allow the camera to sense the presence of a DX-format lens and change from FX to DX mode automatically.
The only change you would see in the viewfinder is a black rectangle appears when the camera is in DX Mode. What oldhippy did was set the camera to DX Mode (not the Auto version), and shot in that mode. He was basically seeing an FX image in the viewfinder, but the camera, being set to DX, only recorded the center portion of the sensor.
Because of the possibility of doing something exactly like what oldhippy did, I have gotten into the habit of returning my cameras to what I call a "Ready State" when I'm done shooting. Any settings I changed to take the shot(s) are undone, and when the camera is not being used, it is ALWAYS set to the same settings.
This way, I know
exactly how the camera is configured when I pick it up next time. Right now, since I'm not shooting anything, both my D600s are sitting in this 'Ready State", both set up with the same settings (save for one will record images with a D61 prefix, the other D62).