“Avoid the passive voice.” updated

When I teach technical writing for my statistics students, I’m always a bit uncomfortable with the dictum "avoid the passive voice;" it doesn’t get to the essence of what I’m trying to say.  Mark Lieberman at Language Log has explained the problem and its solution far better than I ever could: Avoid construction that is vague as to agency.  My latest example comes from a news report about the Dog Psychic, who claims pets form sentences.  OK, but when you get home and the slippers are all chewed up, the dog says "Mistakes were made."

Update (18 March).  Mark Lieberman piles on: