In one of our webinterfaces you could swap between users once you logged in by changing the user in the link.

Not intended but our support is using this "feature" frequently because then they don't have to log in again when helping another client so it's not planned to be fixed

When you try to fix an error for hours and in the end you manage to fix it but the way you fixed it shouldn't work at all.

When you install a new OS on your machine and when you're finally done you make a stupid mistake...

and BOOM!
easiest way to fix the outcome is to redo the OS again...