Ex UK Post Office staff tell inquiry of stress of IT scandal (opens in new tab)

I dont understand it fully. Is it really a problem to counter proof with logs and times when what and how much has been taken out of the cashier? I'm used to think that everyone is not guilty until the guilt it proofed. So, I would like to ask why this obviously didn't happen here? Are the people stupid or am I?

Personal testimony from some of the 700 people who were falsely accused of theft of Post Office funds between 2000 and 2014. The issues were ultimately found to be the result of bugs in a newly deployed accounting system. Posted as a reflection on the real world impact of errors in technology systems.