On 24/02/2018 20:37, Mike Fleming wrote:

programmers
who inherit dreadfully-written [1] systems also don't have a clue how
they work, they just know that they almost always do.

[1] ie. any system composed of programs not written by them

Back in the 1960s, I inherited some of them, poorly documented and
almost impossible to follow the logic because various conditions in past
records had set markers which following records checked for and took
alternate paths depending on what was set and what wasn't.

On the day that the output was noticeably wrong, it proved impossible to
work out what records had set what conditions that screwed up the
processing.

After trying to dry run it for a couple of hours, I decided that it
would be quicker and easier to rewrite the program from scratch, which I
ended up doing.

Jim