I always thought my oven ran hot, but a $5 oven thermometer from the hardware store showed it was actually 25 degrees off. | Baking Fails And Wins - Opinions Matter
My old oven back in my first apartment was like that. The dial said 350, but it was really more like 375. I burned a lot of cookies before I learned to just set it lower. Sometimes that blind trust works out, but man, it can really set you back.