Something very much like that happened to me once.
Though not at the time...
This was during the late 80s when I was Chief of the Remote Systems Branch (basically that meant any computer system that wasn't a mainframe) and the Internet was still relatively young. We used an email program called "em", and its behavior was not always what one might wish.
Our Director in those days was an Air Force officer who had been trained as a meteorologist and whom I'll call Colonel B. What a meteorologist was doing as the Director of the Office of Telecommunications and Information Systems is another story.
Anyway, Colonel B. emailed me a message that contained a very rare (for him) note of praise for something our branch had recently done, and I forwarded it on to Joe, whose section had done most of the work. I included a short covering note, something to the effect "Can you believe the Colonel is actually saying something good about us for a change?"
The following morning Colonel B. called Joe and me into his office first thing. We had no idea what to expect, but I guess we vaguely thought he might still be in a good mood from the previous day.
When we got there, Colonel B. was very solemn, as serious as I had ever seen him.
"I received a message that I don't think was meant for me," he began. "In fact, it might even be disrespectful." (For Colonel B. being disrespectful was about the most serious crime one could commit.) He went on in that vein for some little time, and I was frankly mystified as to what he was talking about.
He finally concluded with some variation of "go and sin no more" as he handed us a printed copy of the message in question. With that, we were dismissed.
"Joe, do you have any idea what he was talking about?" I asked as we walked away. Joe was busy reading the printout that the Colonel had handed us.
"This is the note I sent you last night," he said, "but how did the Colonel get a copy of it?"
After scratching out heads a bit, we finally figured it out. Joe had replied to the message that I had forwarded to him, expecting that the reply would go to me. But our email app, "em", had a lovely little feature that some might call a bug. When one replied to a forwarded message, the reply didn't go to the forwarder but to the writer of the original message. So Colonel B. had received Joe's reply to me.
What did it say? I no longer recall, but it was fairly innocuous. On the scale of disrespectful it was maybe a 1 out of 10. We had each been way more disrespectful in any number of other messages we had exchanged.
But we did learn a valuable lesson about email that day.