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

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 1.3% – although many others claimed to have been affected.

Many businesses would be pretty happy with a 98.7% success rate for its core software – but all it takes is just one of thousands of otherwise successful transactions for each of those 150 people to have had a problem, which would mean an even lower failure rate.

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.

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Hi - just been pointed out to me that your calculation above is a bit wrong (an intermittent fault no doubt)

150/11500 = 1.3%

Cheers, Tim

[You're absolutely right Tim! Thanks for pointing that out. Unexpected and unforeseen user error - see how easy it is! Corrected now. Bryan]
This comment also comes from Tim McCormack, and was sent by email and republished here:

Thanks Bryan - I hate to nitpick but just like the Post Office you have not removed the error completely.

More to the point though. Last week the transcript of a trial (Regina v Misra) was published. Ms Misra, a former Subpostmistress was found guilty of theft and received a prison sentence.

The prosecution case was riddled with errors and the defence was even worse.

Summing up to the Jury the Judge stated "Do you accept the prosecution case that there is ample evidence before you to establish that Horizon is a tried and tested system in use at thousands of post offices for several years, fundamentally robust and reliable"

Why is it that even the Judiciary believe that just because a computer systems behaves correctly most of the time that it can be held to be 'robust and reliable'.

Computer Weekly has exposed the truth. Horizon is not robust and is certainly not reliable. The error that you describe is so similar to another error that was used as an example in the Misra case. It looks almost certain that Fujitsu have never found the cause of the error and it is being repeated throughout the system.

It is most important to note that the error happens randomly. It is an intermittent fault that can and probably does affect more than one branch. It may be only when the amounts are significant that a subpostmaster can identify the error occurring. Sadly though the computer does not generate errors, only when the amount is greater than £1,000.

It is damning evidence of the whole Horizon fiasco, that despite your original report regarding the error, POL have failed to notify Subpostmasters of its existence and to warn them to look for the signs of the error. Meanwhile Fujitsu are not prepared to fix what they perceive to the problem until next March. Why is that?

In my opinion there will ultimately have to be a judicial inquiry and I really do fear that criminal charges may be brought against those responsible for sending innocent people to jail.