The Post Office prosecuted a postmaster for stealing - even though its own criminal investigator couldn't find any evidence of theft, the BBC has learned.
Jo Hamilton is one of dozens of postmasters who were prosecuted after having problems with the Post Office's "Horizon" computer system.
But documents obtained by BBC Panorama reveal it now accepts the losses at Mrs Hamilton's branch were probably caused by "operational errors", not theft.
The Post Office denies any wrongdoing.
It says the Horizon system is robust and there is no evidence that miscarriages of justice have taken place.
Mrs Hamilton was accused of stealing £36,000 from her branch in South Warnborough, Hampshire, in 2006.
However, the documents seen by Panorama show what the criminal investigator reported to the Post Office at the time.
The investigator wrote: "Having analysed the Horizon printouts and accounting documentation, I was unable to find any evidence of theft or that the cash figures had been deliberately inflated."
Mrs Hamilton says she found Horizon difficult to operate and she couldn't understand why the computer kept showing cash missing.
"I rang the help desk and said I'm £2,000 down and she said well you can do this, this, this. So I did exactly what she said and it doubled, so then I was £4,000 down," she said.
Under her Post Office contract, Mrs Hamilton had to pay back any shortfall. She eventually stopped putting in her own cash and signed off the accounts anyway - without officially declaring the missing money.
After the Post Office discovered the £36,000 shortfall she was charged with theft and false accounting.
Mrs Hamilton says she was scared of going to prison so agreed to pay the Post Office the missing £36,000, pleaded guilty to false accounting, and also agreed not to blame Horizon for the losses.
In return the theft charge was dropped. She was given a 12-month community order.
Six postmasters jailed
The plea deal kept Mrs Hamilton out of jail, but she says she lost her livelihood and her good name.
Her case is one of 20 that are now being reviewed by the Criminal Cases Review Commission to see whether miscarriages of justice occurred.
Six postmasters have been jailed.
The Post Office says it cannot comment on individual cases due to confidentiality, but that a financial loss and false accounting together is often sufficient evidence for a theft charge.
In 2012, the Post Office appointed a firm of independent experts, called Second Sight, to look at postmasters' complaints about the Horizon system.
Second Sight has now questioned whether the Post Office may have used theft charges to put pressure on postmasters like Mrs Hamilton.
In his first interview, Second Sight director Ian Henderson told Panorama: "What was of interest to us was that a number of cases also started with an additional charge, which was that of theft, but in a significant number of cases, that theft charge was dropped in response to the defendant pleading guilty to false accounting."
The Post Office says it complies with all legal requirements and that it only prosecutes where there is a realistic prospect of conviction and never for making innocent mistakes.
A lot of glitches
Panorama has also spoken to a former computer technician who used to help maintain the Horizon system.
Richard Roll told the programme errors were far more widespread than has ever been reported.
He said: "There was a large team employed there - 30 or so of us - and we were all full-time and we were all pretty busy.
"So there were a lot of errors, a lot of glitches coming through."
The Post Office says only a tiny proportion of postmasters have complained about the Horizon system.
In a statement, it says: "The Horizon system is both effective and robust. It is independently audited and has been used by nearly 500,000 people since it was brought into service."
"There is overwhelming evidence that the losses complained of were caused by user actions, including deliberate, dishonest conduct."