That light will go off sometimes before a bulb goes completely out...

That would explain why last week when I asked a friend if all my brake lights were working, he said yes. When I looked, one was out! You can pick up the bulbs cheap at any parts store. Be aware that there are two different bulbs,
though they look alike. (One has two "nipples" on the end of the bulb, and the other only one...sorry, I don't know the technical term!)