I've just noticed that whilst shooting with my D5100 using M mode settings reset after each shot.
Say, the camera is on a tripod pointing to an object. I've balanced out the exposure to a 0, aperture set to F/8 (this doesn't change btw) and the shutter speed is set to 1/15. I take the picture (camera hasn't moved) and the exposure level and the shutter speed have changed to some random settings (+2 EV exposure and 1/250 shutter speed - it's different every time though).
I've searched everywhere, but couldn't find an option that would allow me to save/lock my current settings providing the the image in the view finder doesn't change.