"There is no direct evidence of her taking any money [...] She adamantly denies stealing. There is no CCTV evidence. There are no fingerprints or marked bank notes or anything of that kind. There is no evidence of her accumulating cash anywhere else or spending large sums of money or paying off debts, no evidence about her bank accounts at all. Nothing incriminating was found when her home was searched." (The only evidence was a shortfall of cash compared to what the Post Office’s Horizon computer system said should have been in the branch.) "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?"
My word against yours wouldn't be enough to meet the standard of "beyond a reasonable doubt", but the Post Office's word backed up by a computer system? It seems that was convincing enough for the jury. They gave a guilty verdict in the above case.