Last time we went to Paris, was a mixed couple group of us, total rip off, hated the place.
At one point we were sitting at Bar by the River, debating were to eat for the evening, my friend John who speaks some French was very specific when ordered a round of drinks as being slightly diabetic only drinks Pilsner type Beer himself, needless to say He was given what the Waiter wanted to give him, normal Draft Beer with the rest of us. With that John then proceeded to empty the contents of the glass over the pavement, and told the Waiter what He thought of Him (think John was having a bad day).

A local guy was sitting on his own a few tables away, got up, and started rambling on in French to the Waiter, He then turned round walked towards John and produced a Warrant Card, turns out He was Off Duty Police, we all thinking 'this is not good thoughts', the Policemen explained to John, in very good English, that it is not customary to chuck drinks on the pavement in Paris, but then said John's French was very good and explained exactly what He wanted and was very clear on this, He then apologised for the Waiters ignorance and insisted that He wanted to pay for the round of drinks himself.
Needless to say we were all a bit gob smacked but after a while accepted the Policeman's kind offer, after a few more minuets with the guy, He told us where and where not to eat, told us to enjoy our evening before walking off with a friendly wave.