Horizon is the case that should replace Therac-25 as a study in what can go wrong if software developers screw up. Therac-25 injured/killed six people, Horizon has ruined hundreds of lives and ended dozens. And the horrifying thing is, Horizon wasn't something anyone would have previously identified as safety-critical software. It was just an ordinary point-of-sale and accounting system. The suicides weren't directly caused by the software, but from an out of control justice and social system in which people blindly believed in public institutions that were actually engaged in a massive deep state cover-up.
It is reasonable to blame the suicides on the legal and political system that allowed the Post Office to act in that way, and which put such low quality people in charge. Perhaps also on the software engineer who testified repeatedly under oath that the system worked fine, even as the bug tracker filled up with cases where it didn't. But this is HN, so from a software engineering perspective what can be learned?
Some glitches were of their time and wouldn't occur these days, e.g. malfunctions in resistive touch screens that caused random clicks on POS screens to occur overnight. But most were bugs due to loss of transactionality or lack of proper auditing controls. Think message replays lacking proper idempotency, things like that. Transactions were logged that never really occurred, and when the cash was counted some appeared to be missing, so the Post Office accused the postmasters of stealing from the business. They hadn't done so, but this took place over decades, and decades ago people had more faith in institutions than they do now. And these post offices were often in small villages where the post office was the center of the community, so the false allegations against postmasters were devastating to their social and business lives.
Put simply - check your transactions! And make sure developers can't rewrite databases in prod.