The only ones I know of aren't really "horror" stories, more like "slight inconvience" stories. Two happened to us...
1) Our first offer went out and took two days before the seller got back to the reseller and told her they decided not to sell. Not exactly a horror story, but one thing you should be aware of in buying resale is that the seller can back out.
2) Our second offer went through and we eventally closed. But it took a lot longer than we had anticipated. It was only a few weeks (three or so), but if you are in a hurry to close to make reservations, it can be not good.