For what it's worth, I often find myself defending the passive voice in various contexts. I don't know why the Style Tsars decided to hate in that one, but it's really not universally bad.
Most of my complaints about excessive/inappropriate use of passive voice are when it's used to obscure a point. "It was decided that we'd do XYZ." rather than "Pat decided that we'd do XYZ."
Mistakes were made.
Excellent example! This sentence contains no actor at all, unless the mistakes made themselves.