How could a hot-cold plumbing crossover spontaneously appear?
Power cycling a tank less gas water heater? Yours must have electronics that I haven't seen on the models I have installed. There are demand valves in the water heater that sense the flow. These valves can become sticky with lime &amp; rust scale buildup. The reason to turn the water off then open a hot water faucet is to make a drastic change and hopefully the the scale will break loose.Most faucets don't have mixing valves these are usually found in shower / bathtubs that could cause part of this problem. I believe it is more likely the water heater is not turning on correctly or sensing the flow. Some units can be cleaned by disconnecting and pouring CLR (Calcium,Lime &amp; Rust remover) into the inlet and flushing with fresh water. If the sensing valves directly regulate the Gas flow like on a Bosh aqua star you need to get it cleaned out before the seals fail because they are not repairable and the system will have to be replaced.