Ok, this is a fun one. Gone all day on adventures, come home to take a shower. Water on, adjust temp, good. Crawl in begin scrubbing, all of sudden the water gets cold (er). OMG, water heater not working? It's ok. What happens is the water sits in the heated hose and gets warm. So as you run water through the system it stays warm, until you get ground water, then it cools.

The warm water in the hose keeps the shower water temp up. When you start feeding non-warmed water through the system you get the same sensation as the hot water supply running out. It'll get your attention, but it's not a problem.