Well, I just did the same thing for the same reason - I didn't realize it was flat - and now I have a nice new tire back there. In my case a roofing nail made the problem worse. It was right near the juncture of the tread and sidewall. By driving flat, the point of the nail was allowed to create a second hole in the sidewall - which was the kiss of death for that tire. Fortunately, or not, the tires were pretty new, so the size difference is small enough that no one seems to be too worried about it. Getting one new tire is a whole lot easer to swallow than getting four!