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
change the fill valve.