The law used to say 'follow the money'. So where did it go?

In some cases it went to the customers of the Royal Mail because the withdrawal didn't register in their bank account, in others it went to Royal Mail. Maybe, just maybe the error would not be spotted until a full audit of stock was made, and that would be several days later. Remembering a specific transaction amongst 100 would be impossible without the clues.

Some of the errors between two computers/terminals might be errors outside the software's domain **. ie hardware. Did they all use battery back-up? If not, the system design is at fault.

** but within the software's need to cope with it - ie fault tolerant

I well remember the Lundun Ambulance emergency phone service in the early 90s. The winning bid to provide came in at 2 million GBP, but "Tandem Computer" bid 10 (clue in the name, they know about fail-soft). The first winter was cold, and 'flu struck. The service hit its limit and chaos ensued. Why oh why does the world think the word redundancy is about shedding waste. The word means insurance/backup/contingency. Like car insurance. Perfect drivers like me don't need it, but why do I take it out? Yea unforeseen circumstances.