The KC-10 cannot refuel certain aircraft due to the center engine. its great for fighters and bombers but not T tails and awacs.
What are the aircraft that the KC-10 "cannot" refuel? Is that "cannot" definitive or is it supposition on your part and what exactly is it about #2 that makes refueling certain aircraft impossible?
http://www.jetphotos.net/viewphoto.php?id=5893450
or a Tristar and a Sentry AEW.1?