Ever have this happen? You draw a selection with the Rectangular Marquee tool (M) and the corners of your selection are rounded, rather than nice and straight? This happens to a lot of people, especially if they've been drinking. If you haven't been drinking but you're suffering from rounded-corner selections, look up in the Options Bar, and you'll see a field for Feather. Chances are there's some number other than zero in this field, and what's happening is every time you draw a selection with that tool, it's automatically feathering (softening) the edge. What probably happened is you intentionally (or accidentally) added a feather amount at some time, then later forgot to set it back to its default of zero. So to fix it, just highlight the field and type 0 (zero). Incidentally, this is a great Photoshop prank to play on co-workers, friends, soon-to-be-enemies, etc., because the Feather field is usually the last place they'll look.