One company I worked at, some years back now, whose name I've redacted, to save the blushes of the guilty, used public IP addresses on their internal network - a recipe for disaster. With assistance from a good friend, we managed to dodge that particular bullet.
This is even worse :)
Remember when the telephone company came to your house to hook up your phone and gave you a new phone number? This new number was how your friends and family were going to contact you. You counted on the telephone company to ensure that someone hadn't already been issued that number, because if they had, various problems would ensue. What would happen when your mom tried to call your number if it was also assigned to someone else? Could you directly call the other party to work out the problem? Well, in the BGP realm, something similar has been happening with autonomous system numbers (ASNs).
Organizations need an ASN to run BGP and route on the Internet. They are each assigned globally unique ASN(s) by their local Regional Internet Registry (RIR), who get them from IANA. A few weeks ago, the NANOG folks noticed that AS1712 had been registered by two different organizations (in France and Texas) that were both using the number to announce their separate network prefixes. ARIN issued a statement conveying that they were aware of the problem and were working to resolve it. We took at look at the data and found that AS1712 isn't the only dually-assigned ASN out there. In fact, even a root server didn't escape unscathed."
(Via Renesys Blog.)