Yeah, apparently this happened to a jewelry project on Kickstarter a while ago - one of the big backers turned out to have used a card without enough funds and the creator wound up on the hook for all the rewards, but without enough money to make them.
It seems like this scenario could be prevented even within the one-week pre-auth limit cited above. That is, pre-auth every donation a week before the campaign ends. The pre-auths that fail would then be subtracted from the total, and a week is long enough to either pay your credit card bill or decide that you can't contribute that much.
