The scenario: Prudence had stopped at a fishmonger van to buy a kilo of silver cobbler. Selling the fish was a man d'un certain age who wasn't having a good day.
Prudence had handed over her cash, politely declined a plastic bag and was waiting for change.
Fishmonger: Have you had a good afternoon?
Prudence: Yes, thank you, I have.
Fishmonger: Then why aren't you smiling?
Prudence is quite proud she managed not to growl through her gritted teeth, but in hindsight wishes she'd said what she was thinking: that serve of condescension has just ensured I will never buy fish from you again and I will tell everyone I know that you are a patronising git.
Sometimes it is better to set aside good manners for the greater good.