When cold water flowing into toilet cisterns from the main line hits the surrounding warm air of the bathroom, it causes condensation. The toilet cistern or tank actually looks like it's sweating. While this isn't necessarily bad in itself, the resulting run-off can rot out the floor around a toilet. If the water never stops running, it never has a chance to warm up, which means there may be a leak.
Checking Condensation Level
If the toilet tank seems to sweat, there is condensation occuring. To check for a leak, put a few drops of food coloring into the tank. If you see it in the bowl a few minutes later, a leak is not letting the water steady. You can also install a temperator valve to add a mixture of warm water to the tank or use a drip pan to collect the run-off.