I used to work as an automation engineer (basically a programmer for factory machines). Some of the software we wrote had to be password protected. Clients wanting to be able to choose who gets acces and whatnot.
Sometimes however we'd get software from a client where they provide the password so that we can update the software. When my job was done I'd just put in the same password so the protection didn't change. This particular client was one I knew well, having done multiple projects for them so I had a list of their common passwords.
When I quit (multiple reasons, won't go into it here) I heard that the chief engineer on the client side did as well. Didn't think anything off it at the time untill 5 months later. My former boss contacted me through asking for a password for one of their machines. That list I had? Wiped since I had to hand in my work laptop. The chief engineers list? Well he's gone too, so his laptop was wiped as well.
Oh the smile I had on my face being able to tell my former boss that of course I hadn't bothered to remember the passwords. I no longer worked for them so why would I? Never knew if they managed to figure it out, but oh well. It's not my problem anymore.