While its certainly possible to ferment below 1.000, its fairly rare for beer, especially an extract batch. Just for reference, what was the recipe and yeast you used? If there was also a lot of sugar, or you decided to use a big saison yeast, it could maybe get down that far. Otherwise, assuming the measuring was correct, then infection might be the next place to look, since some wild yeasts can ferment the sugars normally left behind by brewer's yeast.
Before you worry though, just wanted to check a few things first. Since you say it was bobbing, I'm assuming you put enough sample in your testing jar (hey, I've made that mistake before
). Did you let the sample sit for a bit before testing it? If it had just finished fermenting, it might have a fair amount of CO2 in solution that could have nucleated on the hydrometer, causing it to rise. If you let it sit for an hour or so (or shake it and then let it sit), you can get some of that CO2 out of solution before measuring. The other thing to check is to make sure that the hydrometer isn't resting up against the side of the container. Since it sounds like you've done this before you've probably already done that, but just trying to come up with options now.