Manners I have been taught that I will never ever need to use again, but did once:
1. When seated at the start of a formal dinner, it is not acceptable to light a cigarette before the Toast to the Queen/King
I learned this at a dinner in Ottawa where we were all waiting for the head table to be piped in and start the pre-dinner ceremonial things. I was about 25. As I was about to light up, an older friend leaned over and shared this tidbit of etiquette with me. I was grateful and pissed off all at once.