The main reason you may have hot water in your toilet is a bad flapper in the toilet tank or a bad fill valve. either one of these can allow the water to seep into the tank. when you have a mixing valve on your toilet to prevent condensation, some hot water is introduced into the cold water line to warm the water just a little, but when the water just seeps a little at a time the hot water dominates the cold and you get a tank of hot water. The solution: replace the flapper and/or change the fill valve.
Other possibilities are, you have a cross connection in your water system.