Not Using the Right Rack
The rack you use for your baking dish is just as important as setting the oven at the right temperature. Different racks in your oven allow for more or less heat to be passed through the baking pans. Your recipe will let you know which rack to use, but when in doubt, stick with the middle one.
Using the wrong rack is a mistake that will lead to either burned food or undercooked food whenever you take it out. Pay attention to your timer and your rack placement every time you bake. There is a reason placement is listed on the recipe, so follow it.