Edit: Based on response, I needed to clarify a few things...
I was recently trying to drain my hot water tank. I turned off the cold supply, opened up a few hot faucets, started draining, but it seemed as though it was taking forever, much longer than the 40-gallon capacity would need.
Convinced something was wrong, I detached the hot line, and then turned the house supply on - WITH THE TANK SUPPLY OFF - and cold water came gushing through the normally hot outbound side! Turned it back off, no more water. For a time, I became convinced the builder's plumber put the tank's supply cutoff on the wrong line.
With the house water turned off, the tank drained in just a few minutes. When I detached the cold line, then restarted the water, the cutoff line held (confirming that valve was not leaking), and cold inbound water continued to come out the normally hot outbound side.
I'm no plumbing expert, but the only thing I could infer is that supply water is jumping onto the hot side, and that would presumably only occur where the two would physically mix, such as in a single-handle faucet cartridge/pressure balancer. Is there some other possibility I'm missing?