I think mine has gotten stuck ON. The day before I left for the party, I was up late (thank god), and had Matt's frag floating in the tank. I decided to check the temp, to make sure it was warm enough for the coral until I took it to him the next day, and the temp was running at 85!! I tried unplugging the heater, and letting it cool completely, turning it down, then plugging it back in, but even though the water was still running hot, it turned RIGHT back on. Luckily, I had an extra cheapie Petco heater I could throw in. It is a little small for the tank, but it isn't frying my fish.
I am going to test the heater in a bucket before I chuck it. I will make sure I post a thread if it is definitely stuck ON.