The fundamental problem with program maintenance is that fixing a defect has a substantial chance of introducing another. So the whole process is two steps forward and one step back. Why aren't defects fixed more cleanly? First, even a subtle defect shows itself as a local failure of some kind. In fact it often has system-wide ramifications, usually nonobvious. Any attempt to fix it with minimum effort will repair the local and obvious, but unless the structure is pure or the documentation very fine, the far-reaching effects of the repair will be overlooked.
— Fred Brooks