I have not actually posted something by mistake however I once sent an email to the wrong person, and I wanted to die.
Of course the email was ABOUT the person who I sent it to and as soon as I pressed SEND, I realised who I had sent it to and panicked and turned off the computer instantly however the email got through of course......it was so, so embarrassing -since then I am so very careful and luckily have not done it again.
A long time ago, when internet was "new", someone else posted my photo online, without my consent. It was something I got in trouble for. But sending emails to the wrong person happens quite often. Nothing damaging though.