Accidental swallow of tap water in the shower.

To be completely safe, try to keep your mouth firmly closed while washing your face in the shower. If you do ingest some, do not panic, just drink plenty of purified bottled water afterwards to flush your system.

It is much more important to rigorously use bottled or filtered water when brushing your teeth, as that involves directly scrubbing your gums and swallowing small amounts of water repeatedly.