What do you do if you accidently turn on the hot water on Shabbos?

Shalom, Thank you for your question. In the work Shmirat Shabbat KeHilchatah (Chapter 1,42) it states "Should one leave on, or mistakenly turn on, the hot water tap of a boiler or heater in which water is being heated on Shabbat, one may not turn it off again on Shabbat, if the hot water pouring out is being replaced by cold water. The reason for this is that stopping the outflow of hot water automatically cuts off the continues inflow of additional cold water and this, in turn, results in the heating up of the cold water which has already flowed in". In the latest Hebrew edition it adds that one is allowed to hint to a non-Jew to turn the tap off. (See Responsa Tzit Eliezer, 18,32). If the hot water system is one where the turning on and off of the tap directly turns on and off the electrical or gas heating unit, it is all the more so forbidden because of the problem of extinguishing a flame, or turning off electricity. Again, one may hint to a non-Jew to turn the tap off. Blessings.