Hello,
you gave the example of legacy systems not being well documented.
Sometimes it can be the other way arround unfortunately. A change of management, a change of IT-companies, and a wreckless import (time you know) of originally more or less well-documented data into a poorly documented "new" system. My heart broke and now two years later there are still all kinds of little tiny problems. But hey, the system runs ! (but the data s**ks)
Greetings,
L