You build a new data backend for a service, you test it a million ways, and finally you migrate all the user's over.
Except it goes wrong. 999,999,999 user accounts migrated. 1 account failed.
Leaving the old infrastructure up for that one user is costing millions of dollars a day, hurting performance reliability and failover and holding up hundreds of other engineers projects.
Turns out the failure is because the user managed to create a filename with 67000 byte order marks in and nothing else, and it trips up some parser in the new system. They probably did it using a custom client or some kind of exploit checking tool. It's probably some security vulnerability researchers test account, but we can't be sure because we can't look into the account for privacy reasons.
Yet fixing the bug for that 1 user account is going to take a while - a quick bodge solution breaks other user accounts, and the proper fix is part of Jane's project due next quarter.
The decision is made. We're going to treat the needs of the many over the few. We're going to disable this one user account and move on. That user will have to re-upload their data, and this time they won't manage to upload a file with 67000 byte order marks in the filename.