I'd imagine something like a guest bathroom that is rarely used so the bacteria have time to multiply and diffuse up the water pipe. Or develop a biofilm on the pipe walls that isn't removed by running water (to see this effect leave a dish of water out for a few days so it feels slimy then try to rinse it with water)

I don't think this is likely at all, but I could understand how it could come about in some niche situations.

You can have pressure loss in the while system and it can suck water up it and contaminate a lot of fresh water.