...that they don't dare say to the nearby adult for whom the message is really intended?
Mum in playgroup whose 4 year old is grappling with my 1 year old for possession of a toy: "Never mind, Barnaby, the little boy obviously hasn't learned how to share"
Random stranger on bus, to baby: "Oh dear, what nasty person made you go out with no gloves on like that!"
Worse still though are the emotionally blackmailing ones to kids old enough to be confused by them...
Elderly relative to my 7 year old on our monthly visit: "I hope you won't take so long about visiting me next time (injured sniff)"
Relative to my daughter: "When Mummy has the baby, can I come with you when you meet her for the first time?" (to which sweet innocent child says "Yes of course you can" while annoying relative, who has been asked to give us a few hours after the birth before they descend, shoots triumphant glance at me over child's head)
I hate sneakiness of all kinds, but trying to implicate children in sneakiness really ticks me off.