Sometimes it happens. Don't worry—you are not alone.
You pull up to the gas station and can't remember which side your fuel door is on. Or maybe you took your second family car (that you rarely drive) to work instead and you need to fuel up before going home. Or you have a brand new car or it's a rental. Take a look at your instrument panel—most cars have an arrow next to the fuel icon that indicates which side the fuel door is on.