There can be differences between gas and electric ovens, and some are off at varying temperatures. Many older apartment ovens are notoriously finicky. Our old 1959 oven was a champ - once you adjusted for the 100-150°F variations.
Most oven thermometers are cheap. You can find them in the grocery store, hanging with the tools. They're also tough - ours is splattered and stained but it works. See more on our favorite thermometer below, and tips for getting the most out of it... We like this Sunbeam Oven Thermometer ($6, Amazon) because of its big colorful dial. Our old thermometer was a little hard to read at a glance sometimes, but this one is bold and bright. Like most most oven thermometers it has a hook and a stand so it can hang from a baking rack or sit on a shelf.
When you buy a thermometer or move into a new kitchen, test the oven's temperature in different corners and high up and near the bottom. Sometimes you find a hot or cold spot that can be adjusted for when baking.
Do you use an oven thermometer?