Mostly non-malicious example... My employer asked me to write a UI to solve a problem for a handful of people until a proper (giant ever-delayed) migration was finished. Over a couple of weeks I made it work despite not having dealt with MVVM/XAML/Whatever before and I was pretty pleased with the outcome. But it was a hacked together thing! I'm not a real dev and given that I got a promise it wouldn't get distributed.

So, you know, in the program.cs startup I checked the username vs a hardcoded list of people in the relevant teams, and if it wasn't crashed out with an error and a support email address.

About 18 months after I had moved on, I got an email with a screenshot of that error message. it would appear the Milan (something like that) office had got their hands on a copy but it just wouldn't work for them...

Trivial to undo of course, but I did enjoy the throwback!