Being the time of the year where family portraits is the preferred shooting mode, I have been shooting for the last two days with my tripod. However, when mounted on the tripod (in P mode), the Nikon D90 does occasionally take the liberty of releasing my shutter at a slightly lower (set at minimal 1/50 in my menu) speed of 1/40 second! Hmm...how interesting!
O yes, this will happen when the "Lo" indicator is displayed in your viewfinder. Rationale? In Program mode, if you do not pop the built-in flash up, the camera will first attempt to raise the ISO. Secondly, it will also attempt to shoot at the largest aperture possible from the lens. Third, if what's done is still insufficient, it will attempt to lower the shutter speed to the predefined lowest. It should not go any lower as I experienced above. I think it can be said that this could possibly be the first discovered "bug" in the firmware?
Nikon D90 still allows a shutter release despite the "Lo" setting but should not "override" the predetermined lowest shutter speed in the Shooting Menu.