Result: Food cooks too fast, too slow, or unevenly.Ideally, every oven set to 350° would heat to 350°. But many ovens don't, including expensive ones, and some change their behavior as they age. Always use an oven thermometer. Next, be aware of hot spots. If you’ve produced cake layers with wavy rather than flat tops, hot spots are the problem.
SaBrina Bone, who tests in our kitchen, advises the "bread test:" Arrange bread slices to cover the middle oven rack. Bake at 350° for a few minutes, and see which slices get singed―their location marks your oven's hot spot(s). If you know you have a hot spot in, say, the back left corner, avoid putting pans in that location, or rotate accordingly.