We have walked out of restaurants at least four or five times, for different reasons. At a very nice place in Rome they gave us a terrible table, and then it turned out they didn't have the wild boar they're famous for, and advertised outside.
Also in Italy, small town near Venice, a restaurant that didn't have a menu outside turned out to be a seafood only place. Not really their fault we didn't want that, so we did apologize and left a nice tip on top of the drinks bill when we left.
On other occasions, in Stockholm and New York, we have walked out because of rude/very slow/no service. Didn't apologize on those occasions.