It is in the nature of the elderly and/or disabled to forget, sometimes, to put their blue badge clock out when they park on single or double yellow lines (which they are allowed to do for up to 3 hours as long as there aren't any restrictions on loading, which are usually indicated by kerb flashes and a sign).
The mother of a friend put her badge out, but forgot the clock, when she recently parked in High Barnet at 10:38 in the morning. She got a parking ticket.
Mr Mustard appealed on the grounds that although the clock was not on display a contravention could not have taken place as the yellow line is only operative from 08:00 so parking tickets for this offence should not be issued before 11:00
The council didn't accept that argument but did cancel the ticket because of an unspecified "technical error". It would have been nice to know what it was but the main thing is that the parking ticket was cancelled.
Mr Mustard is inclined to think that the argument is a good one. It has been used to good effect in PATAS case 2080110323. Mr Mustard recommends that if you find yourself in similar circumstances that you trot out the same argument.