Oddbean new post about login | logout If a chain split would only cause a divergence between a really old client version and a recent client version, but nobody runs the really old version, has the chain even split? https://blog.lopp.net/has-bitcoin-ever-hard-forked/
I believe a recent error was caught precisely because of this. Matt Corallo knows a lot about it. Maybe it wasn’t exactly a consensus bug, but there was something important in the last few years that was caught because the last version of core was still widely used on the network. I’d have to google to get the details back.