1. Fractional Reserve Banking. For example, MtGox, they do FRB. (I'm wrong? They don't do it? C'mon, even small kids know that MtGox loves FRB.)
credible reports surfaced that ppl could sell more btc than they had at gox, and gox would not give them an error or refuse the order.
that's NAKED SHORT SALE at Mt. Gox!
Yeah, it would be. Except that when that order comes up for execution, it gets truncated to the currently available balance. Also, it hasn't been that way for a long time; now the truncation happens when the order get switched from pending to active.