At a prior job, there was an internal project code system for tracking billable hours or people assignment kind of thing. Everyone knew the codes of their projects. It was a six digit code, two letters and then four numbers: giving you some ~7 million point space. Company was ~100 years old and only had some 15k codes recorded in all history. The list of codes was manually updated once a quarter by an admin who might add another ten at a time.

Some chuckle head decided to replace the system with UUIDs. Now, they are no longer human memorable/readable/writable on paper/anything useful. Even better, they must have used some home grown implementation, because the codes were weirdly sequential. If you ever had to look at a dump of codes, the ids are almost identical minus a digit somewhere in the middle.

Destructive change that ruined a perfectly functional system.