The story goes that a Windsor farmer accosted the King in on eof his fields and accused him of trespass (i.e. crop damage) and got an apology from him.

And, of course, there's William Beckford Snr, Lord Mayor of London, who told the king he was a twit in public; the result of this action being that Beckford's statue was erected in the city (outside the Guldhall? I don't remember that part.)