During the few months after getting my BS and before starting my MS, I worked for a bank that held lots of securities - and gold - in trust for others. There was a massive vault with multiple layers of steel doors, iron door grates, security access cards, armed guards, and signature comparisons (live vs pre-registered). It was a bit unnerving to get in there, so deep below ground, but once in, it looked very much like the Fort Knox vault scene in Goldfinger.
At that point, PCs weren't yet available to the masses and I had very little exposure to mainframes. I had been hired as an assistant to one of their drones who had been assigned to find all of the paper-driven-changes that had gone awry and get their books up to date.
To this end, I spent about a month talking to everyone involved in taking a customer order to take or transfer ownership of something, and processing the ledger entries to reflect the transaction. From this, I drew a simple flow chart, listing each task, the person(s) responsible, and the possible decision tree at each point.
Then I went back to each person and asked them to list all the things that could and did go wrong with transaction processing at their junction in the flow.
What had been essentially straight-line processing with a few small decision branches, turned out to be enough to fill a 30 foot long by 8 foot high wall of undesirable branches. This became absolutely unmanageable on physical paper, and I didn't know of any charting programs on the mainframe at that time, so I wrote the whole thing up with an index card at each junction. The "good" path was in green marker, and everything else was yellow (one level of "wrong") or red (wtf-level of "wrong").
By the time it was fully documented, the wall-o-index-cards had become a running joke. I invited the people (who had given me all of the information) in to view their problems in the larger context, and verify that the problems were accurately documented.
Then management was called in to view the true scope of their problems. The reason that the books were so snafu'd was that there were simply too many manual tasks that were being done incorrectly, cascading to deeply nested levels of errors.
Once we knew where to look, it became much easier to track transactions backward through the diagram to the last known valid junction and push them forward until they were both correct and current. A rather large contingent of analysts were then put onto this task to fix all of the transactions for all of the customers of the bank.
It was about the time that I was to leave and go back to school that they were talking about taking the sub-processes off the mainframe and distributing detailed step-by-step instructions for people to follow manually at each junction to ensure that the work flow proceeded properly. Obviously, more manual steps would reduce the chance for errors to creep in!
A few years later when I got my MS, I ran into one of the people that was still working there and discovered that the more-manual procedures had not only not cured the problem, but that entirely new avenues of problems had cropped up as a result.