Auto Parts: flat battery, door locks, flat battery

Question
I plugged my block heater in last night. This morning, my car battery was completely flat. The door locks wouldnt even work! Anyone know why? The car is  Forrester...i'm thinking maybe a short in the grounded extension cord???

Answer
    I'm just guessing, but did you leave a door or the hood open?  They all have lights that come on when they're opened, and they could have drained the battery overnight if it was just a little weak.  There's no way that the block heater could hurt anything, it's completely isolated from the car's circuitry.