Yep, had the same problem when we bought a PC from Dell, they reserved the money but then never took it (it dropped off a week later) but two days before it dropped of tried to take the money.
They didn't get it as there wasn't £3000 spare on the card and canceled the PC. After a lot of wrangling they finally got it sorted. It's a really bad flaw in the system.
Saying that, it is a phantom transaction so will fall off the end and will never show on you statement (or cause you any charges) but like you said could have caused you to bounce a few DDs!