Insanely high electric bill
Heated bath light/vent left on, if electric furnace, stuck on, or same if heat pump. Now worst case scenario, you just got a real reading, and they are billing you for a mistake. Call the utility to come out and check the meter, also compare previous bills. If you have a swimming pool, the heater was left on, same if you have a jacuzi. If electric water heater, elements possibly went bad. Only you will know by going through and checking stuff out. Get an electrician to help determine what is eating up juice. You can also get a kill-a-watt or TED-5000 to allow you to watch to the detail, your power use.
This Summer has been a bad one. Usually we pay $86 for Summer during the June billing, this year it is $112, due to the heat wave. Hate to see what my July bill will be.