Not all ovens are created equal. Some have room for 5 sheets of cookies, some only have room for a 9x9 inch brownie pan. Then there are those that are 50°F too hot or too cold. Is your oven a member of this club?
To make sure you don't burn your beautiful cookies that you slaved over, buy an oven thermometer. They are cheap and worth it to save you from the stress of dealing with a temperamental oven.
It also doesn't hurt to pay special attention during baking. There may be hot spots or issues with evenly baking a cake when it's on the top rack. A rule of thumb is to start out baking in the center of the oven and turning the pan halfway through to evenly bake.
Photo by ccharmon