To take advantage of the machines, Hayenga said, a student would swipe his Bevo Bucks card as usual. The machine would register the amount of money available, and authorize a purchase. The student would then disconnect the machine's modem, preventing it from telling a central server to subtract money from his or her account.

Shades of this story about the the "Blackboard" system. (I'm sure somewhere in that article or the ensuing comments you can find a link to the actual dirty details.. oh, wait here it is).

Unfortunately for the students, the system wasn't quite as stupid as they'd hoped:

"If you think about how they record the transaction, they probably record all the broken transactions," the student said. "They can link your student ID to the broken transaction." Which is exactly what happened.