GTC. Second-hand, so with caveats, but apparently he put more on the GTC than his TDY actually cost. He was unable to pay off the whole amount right away, so he worked out a deal with his supervisor to pay it back over a couple paychecks. Someone higher up in the chain decided this was unacceptable and had him put up for a court martial for abusing the card. The star piece of evidence was a receipt from a burger place showing he purchased three hamburgers. Since he was buying food for other people using his GTC, they found him guilty.
Turns out the guy was a super-star prior to this, multiple awards and deployments, and the reason he couldn't pay it off right away was because he had custody of his own kids from a previous marriage, and his new wife's kids (total of 5, I think).