This happened to my wife's macbook pro spacebar, $600 fix. It happens with mine every so often too, but fingers crossed it has 'fixed' itself each time.
Had a similar problem (faulty key) with my dell precision laptop. $30 new keyboard, few screws out, replace and back up and running.
The laptop is big, heavy and ugly. But I cannot have it out of action so every part being replaceable was a huge plus. In fact it is so big heavy and ugly that when I drop it on the floor, the floor dents :)