Death by...
Had one such message in an embedded system created back in the early 80's. At a point in the home-brew task scheduler we created, there was an error message we created at the end of a rather complex if-else if cascade that read "Death by <redacted>"* which would scroll across the 8-character 7-segment LED display of the device. We tested the living snot out of it, and were convinced that the conditions that would raise this message really could not happen.
Enter Murphy's Law of Demos.
A potential customer came into the office to get a demo of a beta version of the device. We ran it through its paces, and all went surprisingly well. Up to, of course, the final act of the demo, where, this message started to scroll across the screen. The potential customer had looked away as the message started scrolling, and when we saw the message start, we surreptitiously and quickly rebooted the machine, deftly explaining away the reboot as the result of an odd previously identified beta problem that would not occur in the actual release. The potential customer was satisfied with the explanation, and IIRC, ordered several of the devices.
Naturally, we were subsequently unable to re-create the problem. But, we came to understand that there is not such thing as an "impossible" error, and rewrote the error message to be more "customer friendly". Don't think that the message (in any form) ever did show up on production boxes.
*"Death by <redacted>" was the punchline of a particularly racist, homophobic joke that made the rounds of our dev team at that particular point in time. This was the 80's, and such stuff was tolerated back then (as was smoking in the office...). Nowadays, one could get summarily dismissed for circulating such stuff around the office. So I won't.... (Hence, the redaction.)