As for the water served on aircraft, my understanding is that things like coffee and tea would come from a tank. These tanks are occasionally flushed out. My brother is a pilot on an airline. Sometimes they're flushed and sometimes they're not. There have been inspections in which they found traces of E. coli as well as other microbacterial residue. Also, people will use tap water in washrooms even though there's a sign saying non-potable water. So those are two examples: the tanks for coffee and tea, and water used where it shouldn't be.
