Passive voice isn't always against the rules. There are rare writing constructions where passive voice should be used.
Here are a few of the most common examples:
Mistakes were made.
The identity of the person who made the mistakes is hidden by this sentence. This kind of construction is commonly used by faceless bureaucracies to avoid singling out individuals for blame.
The murder victim was shot in the back four times.
In this example, the author (hopefully) wants to emphasize that the murder was committed by an unknown person.
The control group was given a placebo.
In scientific papers and scientific writing, the use of the word "I" is frowned upon. This one of the reasons why scientific writing can be so painful to read.
Here's a quick quiz: How many instances of passive voice, aside from the example sentences written in italics, were used in this post? Can you find them all?