one time at work i hit reply all instead of forward, and sent an email to my boss (the intended recipient) AND to the client in question.  my email, in reference to the client, said “i think i pissed him off.”
i was completely unaware that i’d sent the email to the client, too, until i got a reply from him a few minutes later, which said “NO YOU DIDN’T HE IS A NICE GUY.”