Sometimes everything just goes to hell in a hand basket, doesn't it? There you are, all set to perform a little minor maintenance, when—bang!—the screw you tried so carefully to remove without stripping does exactly that. In some cases, you may be able to get a bite with a Vise-Grip and spin the bugger loose, but in most cases, once those slots are gone, so is any chance of removing the screw or bolt without resorting to things such as drills and screw extractors or expensive removal tools. But before you panic, try this: grab a small chisel and position it so the edge catches as much of one side of the screw head as possible. Hold the chisel at a 45-degree angle and give it a good whack with your hammer. The object is to use shock to loosen the screw and drive it around, which nine times out of 10 it'll do. If the first attempt isn't successful, don't be afraid to try again, repositioning the chisel if you have to. Naturally, the screw head will be destroyed during the extraction process, but the screw wasn't going to be of much use anyway, was it?