It could be that all global systems happen to rely on some kind of single point of failure that the team never really realized before. Or if they did realize it

It could be that all global systems happen to rely on some kind of single point of failure that the team never really realized before. Or if they did realize it they didn't realize the potential impact. Or it was a code change that was able to be validated and rolled out to all regions but then some totally independent interaction caused it to fail afterward in a unique way. I'm banking on surprising singe point of failure though. I really hope we get a detailed post mortem here.