The garage my Motorhome is in at the moment for its MOT tested one of my self-builds a few years ago and failed it on the numberplate light. once they finished the test, they put a new bulb in and then immediately passed it -so I had a fail and a pass around 5 minutes apart
I asked why they didn't just swap the bulb and do a pass and apparently all these tests are timed so couldn't (plus of course no repairs are allowed as soon as the test is started).
Oh - and no charge for the bulb either.