Baking sheets are kitchen workhorses, but over time, they can warp, stain, and become discolored beyond recognition. Warping happens due to exposure to high heat, which can make them uneven and affect how your food bakes. Stains and baked-on grease are tough to clean and can transfer off-flavors to your food.
If your baking sheets wobble or look more "rust" than "roast," it's time to replace them with a fresh, flat set.