Off the top of my head? One network they multihome had a weird packet loss only experienced by DNS and other services, so they tried to cut the routes over to the second network, but packets were still getting sent to the first network (which had DNS disabled but ICMP enabled on the hosts) and further router fuckage prevented them from switching back easily. Hell, they probably just couldn't get their BGP to propagate once they made the first change.
If you go with 'router tables' being the culprit, they probably had a core router that maxed out its RAM when they added another router in place, but they had already moved a part of the network that housed DNS by the time the routers synced and RAM filled from too many BGP lists to sort. You can still ping 'hosts' (which are I am almost certain a hardware load balancer and not an actual DNS host) while the DNS traffic is going nowhere because the backend DNS services were moved. Would take a couple hours to unfuck all of that.