This happens a lot, and i'd like to say I've found a way to rectify it, but...
One thing you can try, rather than starting again, is to use the shift and alt keys to add/subtract from your dud selection (shift to add, alt to subtract). This is probably the best option if you are working on a large image where you need to zoom to get the level of detail you want to select. You can keep adding and subtracting to your heart's content until you have the selection you wanted.
Darren
