It only had to happen to me once, to have one of my emails forwarded to the last person I wanted to see it.
I write everything with the idea that someone else may see it. If I criticize someone, I do it in words I would use if I was saying it to their face. If I reveal something personal, I do it with the understanding that others may see it who I did not intend. It changes the way you write, but for the better, all things considered.
Everyone has a favorite email faux pas; I had a buddy of mine who was a CSR send an estimate to our boss for approval, the boss replied that the price was okay, along with a string of invective and angry pejoratives about the client and my buddy sent the whole thing along to the client.
My friend, who was just getting his start in agency business and was really nervous in the first place, literally and very nearly had a heart attack.
My boss, who is a confident and wealthy exec laughed the whole thing off.
The client, seemingly, never saw the comments or never said anything