This hiccup in the upgrade should serve as a warning to us to create more mining node implementations...
The fact that a Single line of code can bring our entire network to a halt is a major problem. We need diversity in our network to eliminate single points of failure. It's unfortunate that today that single point of failure was bitcoin ABC.
Edit: I know there are multiple mining implementations, but if that is the case, we need more mining pools diversifying their implementations.
Edit 2: From this post seems this was actually an exploit of the mining code, with a malicious attacker waiting until today to cause maximum effect.