[ali1234]

No, you've not understood what happened.

The SPMs were forced to sign and take responsibility for accounts that were prepared by the Horizon system, which claimed the SPMs owed the Post Office tens of thousands of pounds of imaginary money. If they later claimed that the accounts were wrong (which they were), they'd be prosecuted for signing them, and if they didn't do that they would be prosecuted for not paying the PO the money they supposedly owed. So they would lose either way, and the actual accuracy of the accounts was never tested in court, because in any individual case the PO did not disagree with the SPM about that.

The point being that this can't be blamed on software bugs alone. It was caused by different departments of the PO having completely contradictory ideas about how the software was supposed to work.