About a month ago my stick on thermometer stopped reading. Might be because it was in direct light from the MH. might be because its 5+ years old. Either way, i wasn't too worried about it since my temp rarley moved. My fish where all doing fine, however, some softies where not doing as well. I'd also added a 50W heater as a backup since i took down my 10g freshwater tank. so... while cleaning the tank i realised that the water seemed a bit warm, and i should probably cough up the $5 and get a new thermometer.
Moral of the story... Your arm is not a thermometer.