Come on people
As those engineers who have been around long enough know, no amount of backup or redundancy can save you from that one massive core router that refuses to die in all the traditional senses but nevertheless still fails to do its job. That's how one router causes an outage.
Consider a device that's pingable, responds to SNMP and can even accept traffic but despite being online in every monitored way, still fails to route, drops 85% of its packets and kicks you out of the console every 10 seconds - those, my friends, are the ones that causes f**k-ups like this.
But knowing BT - what they actually mean is a cluster of primary and standby routers went "faulty" because they pushed out a duff config to it and took 5 hours to find out who did what where.