Yep. I’m not an accountant or even a manager, but even I know that. Apparently the Post Office testified many times that the transaction record _was_ immutable,

Yep. I’m not an accountant or even a manager, but even I know that. Apparently the Post Office testified many times that the transaction record _was_ immutable, even though Fujitsu knew that records could be edited or even lost at any time. And it appears that their own customer service employees would edit transactions all the time, to correct perceived mistakes. And it would appear that this might even make the problem worse, as the Horizon system could fail to correctly merge those changes down the line. It could silently revert the edits or silently drop entire transactions as if they had never happened.