In your school, university or other centre of learning, do you ever find (or indeed put up) notes that begin ‘Polite notice’? You see these quite a lot these days, and David Mitchell has talked about how just saying ‘Polite notice’ doesn’t necessarily make it polite. It might actually be quite annoying, or even rude, for example suggesting that you need to be told to ‘wash your hands’, or ‘wipe your feet’, rather than being someone who would automatically do that anyway.
The other thing about these notices is that they seem to play off the possibility of being misread. I saw one the other day (out in the street somewhere), and at first glance, I thought it said ‘Police notice’. As it was about parking (or rather, not parking) this made sense on an instinctive level, but then I looked again and realised it said ‘polite’ not ‘police’. I’m sure part of the popularity of these notices has come from this misunderstanding and the sense of legitimacy it gives the note. That, coupled with the suggestion that if you preface something with ‘Polite notice’, you can say whatever you like, could lead not only to an outbreak of silly signs, but to non-native speakers accidentally causing offence by applying the same kind of rules in conversation.