I have a switch box with 3 switches in it, each powering a different light. All three switches are supplied by pigtails from one incoming cable.
Two switches are simple single pole switches and when the breaker is turned off those lights stop working.
The third is a 3-way switch and when the breaker is turned off, the light continues to work.
All four combinations of both 3-way switches work as expected, with the breaker off.
When the 3-way switched light is lit, it does NOT back-feed the other two lights.
I read online that there used to be a technique used where two breakers/fuses on the same phase were connected through some black magic to power a light, but I can't envision how it works.
Sadly half of the wires are knob and tube so I can't see any neutrals.