(something) was (done)

The person in the example sent the message. So why didn't he write, "I sent it by mistake"?

When you write that "(someone) did (something)", it puts praise or blame on the person you're writing about. For example:

Paul made a mistake in it.

If you don't put the person's name as the subject, it sounds much more calm and objective, even though it means the same thing:

There was a mistake in it.

This phrase appears in these lessons: