THAT should not happen. If you need to run pipe such that
there's a low loop in it, there should be a
drain of some kind at the bottom of it.

I'd wager it happens more often than you seem to think. I'd say the
average house is just not piped such that being able to gravity drain

Sorry, I didn't mean to indicate that it DIDN'T happen,
just that when it does, it's wrong. If there's
a low loop that you need to get water out of, the
correct solution is to cut a segment out, and install
a shutoff with a waste-cap.