My home was built in 1941. The wiring is a mix of original and some added updates. I recently went to wire in a bath exhaust fan and timer switch on the same circuit as the light-switch (they share a box now) I traced the circuit (14) to a nearby box with a ceramic light also on circuit 14.
This small octagonal box had 8 cables coming out of it. Using a voltage tester I determined circuits 10 and 12 also ran through this box. With all 3 circuits turned off I proceeded to separate circuits 10 and 12 into different junction boxes to free up room to tap off 14 for power - and I prefer not to have multiple circuits in the same box for reasons I am about to come to.
The voltage tester indicated voltage was not present, but when I disconnected a group of white wires I saw sparking and lights I know to be on circuit 8 went flickered and went out. I turned off circuit 8 (that's now four breakers off to cut power to this one junction box with light).
After separating all the wires for 8, 10, and 12 (14 was wired to itself and not interconnected like the other 3) I turned the circuits back on and found that both the black and white wires from circuit 8 were 'hot'. There are 3 sets of 3 and 4-way controlled lights on 8, but no switch position checked had any effect on either black or white, they always stayed hot.
Tracing 8 I found that there is a cable going from one junction box back to the panel with the black removed completely (white and neutral only) and another that goes to another junction box down-line on circuit 12 (again white and neutral only with black removed completely from the sheathing).
This wiring has been this way for at minimum 22 years (my father may have added some of these wires between 1985 when they purchased the home, and 1997 when he passed, or the previous owners may have done this) Either way, in 22+ years it has been working, but I haven't a clue as to why it would have been done this way, nor if it's to code.
10 come from the panel to the box (what i thought was down-line circuit 10 is actually fed from a junction box on 8), 12 also comes from the panel, but has two cables feeding down-line.
Attached is a diagram showing the connections that concern me black of 10 to black of 8, white of 10 to white of 12 (not from panel), all 3 blacks of 12 are connected together, and the 'Hot' white from 8 is connected to the white on 12 coming from the panel as well as the white on the other 12 that runs down-line.