It's not. However, I have had all three brake lights fail on one occasion. I know they were working the day before, as my future wife was following me home, but coming home from work the next day, someone got out of their car at the traffic lights and told me that all three were out.
A friend had to be towed home by his father, when all his electrics failed.
I have known of failed brake-light switches - and of the opposite, switches that have broken loose and left the brake-lights stuck on.
Nasty.