About 2 years ago, US Airways had a glitch that allowed negative balances. I was burning some miles for magazine and ordered some. I ordered 1 too many and got a negative balance. I sort of paid them back when I left a magazine on a US flight.

AA allowed a miniscule negative balance (47 miles?) when a relative of mine redeemed an award. However, they got paid back with future flights....and an appreciative relative.