> Come to think of it, I've been baffled from time to time by people

In my experience there are couple of situations where a bright light nearby can
interfere with viewing the planets.

1. The angle of the scope/eyepiece is lines up with the light and causes
either a reflection off eye lens or my eye's lens....

2. When using a Newtonian where the focuser is close to the end of the OTA and
again the angle is just wrong so that stray light ends up illuminating the
eyepiece.

Mostly these are problems with viewing DSOs but can happen with planets..

jon