I saw that a co-worker had left their office email open on their machine, so I typed out a huge hate mail of the upper management and then announced resignation for the poor work culture that the company provided. Then I edited the email to be a bit more nice. I added some praise about the company - about having the opportunity to work in the company and for the amazing colleagues (and mentioned my own name) in the first paragraph. To close the email, I wrote :
"PS : This is what happens if you leave your machine open for the office to do as they please"

I first sent out a copy to myself (as proof) with the cover :
" Hey, check this out, I'm sending this out to everyone@company.com in a while. I want to let you know that none of this is directed at you. You've been an amazing colleague and mentor. You've been my inspiration from the start; from the time I joined the team. I'm honoured that I got to work with you. I hope we can remain friends as we are now, meet up once in a while outside work and discuss life. "

And then I put the actual email up in the compose window with the to field addressed to everyone@company.com. I didn't hit send.

Funnily, enough, this person never found out that it was me who actually typed out the whole email for another 1.5 months. They probably looked into their Sent folder later on when they saw the email that I sent to myself. They replied to it saying :
"Thank you for not sending out that email that day. I've been very very extra careful (I didn't understand the "very, very, extra" part) since that day"
I replied that it was only to prove a point and that I thought the point was well conveyed.

I had a good laugh that day. Since then, every time we crossed paths, we had that look in our eyes that met and only the 2 of us understood.