Erm, not always. It is possible, with some settings on some (usually the more expensive) motherboards to make alterations that cause permanent irreperable damge to the mobo/processor/memory.
Worse (?) on some (particularly laptops) even setting a password is irreversible*, if you forget it.. * other than at a workshop.
There is also the rather more amusing thing to do - set a password using a foreign (eg French) keyboard and then take it elsewhere to use, where the keyboard is different (eg Russian)....