A long time ago driving along minding my own business, was pulled to the side of the road by police. They said did I know my son should be in a seatbelt and then policeman realised my son was but he could not see it. Started waffling about my old car instead.
I asked for his name and number. He said 'Why!' I said 'rear light not working, brake light not working'
He then pretended his radio called him and rushed off. My son reminds me of this quite often and we have a chuckle.
On another occasion whilst sitting in heavy traffic, looked out of my cab and down at a police car and noticed their roadtax was out of date by about a week. Opened cab window and told them, watched by bystanders. Policemen get red faces and zoom off round the corner to their local 'nick.