Software is never perfect - and that includes the Post Office's controversial Horizon system

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Software goes wrong. Every developer knows that. Even the most thoroughly tested piece of software can come up with an unexpected set of circumstances that cause it to behave in an equally unexpected way. Sometimes those unique cases can be so unusual, they are impossible to replicate.

It is difficult to believe that any large-scale, complex software application is entirely and completely free of any possible flaws arising from unforeseen circumstances, no matter how well it performs in the vast majority of usage.

This, essentially, is at the heart of the ongoing dispute between subpostmasters and the Post Office over its Horizon IT system.

The Post Office has consistently said there are no systemic flaws in Horizon, and certainly none that would have caused the accounting discrepancies that led to subpostmasters receiving fines and even jail terms for alleged false accounting.

The organisation has pointed out that affected postmasters are a "tiny" proportion of the number who use it successfully to process millions of transactions every day.

And in turn, that is exactly the point that campaigners make in response - that all it takes is a tiny number of unexpected, unusual circumstances that perhaps cannot be replicated. There are about 11,500 sub-Post Offices in the UK, and just 150 subpostmasters in the Post Office mediation scheme - that's 0.013% - although many others claimed to have been affected.

Most businesses would be pretty happy with a 99.987% success rate for its core software.

The Post Office says, "The Post Office takes its responsibilities towards its postmasters extremely seriously and wholeheartedly rejects any suggestion to the contrary.

"Neither the Post Office nor other parties have identified any transactions caused by a technical fault in Horizon which have resulted in a postmaster wrongly being held responsible for a loss."

And they are correct - none have been identified in those cases. But that doesn't necessarily mean that in 0.013% of sub-Post Offices, there wasn't some undetected, unrepeatable problem that affected Horizon - user error, a power spike, a momentary hardware glitch, coffee spilled on a keyboard.

This week, Computer Weekly revealed the Post Office knows about a recent flaw that can cause accounting errors, and it's being fixed. So it is possible for a problem in Horizon to occur that could lead to a similar situation to that faced by the affected postmasters. But, as the Post Office stresses, there is no evidence to show that it did so in their specific cases.

The lesson for all is that no organisation should assume that its software is perfect.