I have a negative -4,000 miles US Airways balance right now.

I had 26,000 miles. I redeemed a ticket on UA SFO-Hawaii. They should have charged me 35,000, however, they only charged me 25,000 (my guess: Indian call center agent did not understand the destination, which is a smaller airport, was in Hawaii).

Later there was a flight change and I had to have the ticket reissued. The USA call center handled. They didn't say anything to me, but when the ticket was reissued my account showed a redeposit for 25,000 and an issuance for 35,000. This left me with negative -4,000. I flew the flights and everything went fine. The negative balance has persisted for over six months. I am "done" with US Air so this doesn't bother me.