Thanks Joey - you got the head of the nail. In standard UK cables, just like yours, the earth wire is bare copper. It is correct practice to slip a green and yellow insulating sleeve over the exposed part when wiring a junction.
I used to own Norton motorcycle, then a Triumph. I had a black t-shirt that said, “Lucas, prince of darkness.”
But they can create noise by introducing ground loops. Which is why some installations require insulated ground wires.
The three-position Lucas switch: Dim, Flicker and Off.
Snerk.
I don’t think this has quite been suggested; look for a second panel somewhere in the house. If you have air conditioning, for example, that sub-panel could be up in the attic.
We had some work done recently and one of the tasks was adding an outlet in an upstairs hallway. The genius electrician (we got rid of him) wired it into a new breaker in the AC subpanel in the attic. If that breaker tripped, you could never detect it looking at the singe-breakers in the Main Panel.
Disclaimer - I admit I’m not sure if the sub-panel has it’s own breaker breaker in the main panel. It does seem unlikely to me they would run a direct-to-meter line up through the house. But still, if the OP is just throwing single breakers in the Main Panel, this could explain it.
My house was built in 2002, when wiring was cheap and components were expensive. Hence there’s a LOT of wiring everywhere to give things like wall sockets controlled by 3-4 switches.
A few weeks after we moved in, the upstairs master bath socket stopped working. It took me quite a while to figure out that all three bathrooms had their counter-side sockets routed through one GFCI in the downstairs bathroom. It was cheaper to run those miles of wire than pony up for two more GFCIs.
Now, of course, house wire is so friggin’ expensive that builders use optimization programs and will cheerfully throw extra fixtures and GFCIs to save a 10-foot run.
I still suspect it’s a double-breaker condition. Hey, OP, had time to run it down yet?
yeah i’ve found outdoor and garage receptacles fed by bathroom GFCI receptacle feed through.
The trick is not to have those outlets be GFCIs as well (or not feed them from the load side of the other GFCI). Attaching a GFCI to another GFCI just makes them start popping each other and it’s a PITA to track down. It’s how I learned what outlets in my garage where attached to which outlets in my house and it wouldn’t surprise me if it’s how you learned that the outlets in your garage where attached to the outlets in your bathroom.
This, people, is why GFCI outlets come with those stickers that say “GFCI PROTECTED OUTLET”.
When we moved into our last house none of the outside plugs worked. Breakers were well-marked, and seemed fine, so I followed them all around the house and under and saw where they led: to a GFCI outlet in the garage I hadn’t noticed! First homeowner fix for that house: reset it. 
I never mind it when I have to hang up my tool belt with the tools unused.
I checked, and as we suspected, the fixture is controlled by two breakers. I opened both switch boxes and found that one of the two 3-way switches (a motion-controlled switch) seemed to be wired incorrectly. I replaced it with a standard 3-way switch (which was the project that opened this can of worms), wired correctly, but it didn’t solve the problem. It, and the other light in this box, are still on two breakers.
Where should I go from here? (I’d like to try and fix it myself before calling an electrician.)
OTOH, what is the downside of not doing anything, aside from the inconvenience and the annoyance of knowing it’s wrong? Is there any potential danger?
hard to say what the consequences are without seeing how it’s wired. If you have 2 breakers tied together then if one trips the other carries the load of both. But that one should also trip if it gets overloaded.
I would get an electronic tracer and see what is connected to what.
I would shut off both breakers, open up all the light-switch boxes, and determine how power is coming in two different ways. Take digital pictures, so you can always check how the had been wired, then unwire any two-wire romex (2 wires plus ground) coming into the boxes. Separate these wires, so they aren’t touching anything (ETA: and make sure no kids or anyone will touch them), then turn one breaker on, and use a volt meter to find which wire is powered. Turn that breaker off and turn the other on, and find the other powered line.
Now figure out which power lead you want to use, and cap the other one. Rewire everything the way it had been, except for the now-capped line.
Too late to add, but when putting it back together, take some time to make sure you’re switching the hot, so that the lights are connected to neutral when they are off. Since it’s mis-wired, there’s no telling if that was the case originally.
It sounds like you didn’t open the lighting fixture box… the miswire might be in there, too.
I started to give you several ideas of how to proceed but really, there are so many ways to go wrong the best answer is to call an electrician. I can just about guarantee he (maybe she) will sort it out for the cost of the basic service call. An experienced eye will spot the error as soon as they look in the affected box, and from there it’s two minutes’ work… and it will be correct, and safely done, and you can stop worrying about the problem and whether you fixed it right or safely.
Post a report from the pro’s findings, if you go this way. Still curious how it could be screwed up this way.
the danger also is when a person thinks a fixture(s) should have no electricity, because a breaker was turned off, it still would.
A thought occurred to me: Do any of the light switch boxes have multiple switches? Especially one where the other switch is for something other than overhead lighting (maybe it controls a wall outlet)?
If so, start with that box before opening up the others. If two power lines came into a single junction box, I could see someone just tying all the black wires together, and tying all the white wires together, and it all seems to work fine*.
Are the breakers labeled? Which two power the light?
*As an aside, anyone know if that would that be against code? Doesn’t sound like a good idea. (Although, I suppose, what could possibly go wrong? ;))
The first thing to do is to find out how power from two breakers is being supplied.
Kill both breakers, make sure that everything’s dead in the light box plus both switch boxes.
Turn on one breaker and figure out where the power is coming in. Assuming you don’t know how the 3-way is wired, try all 4 switch combinations (down/down, down/up, up/down, up/up) and make a note showing for each combination, where you see power for that breaker.
Then flip that breaker off and turn on the other one, and repeat.
This should give you enough information (plus I assume you know that a 3-way switch is just a DPST switch and what that means) to figure out how power is arriving from two breakers.
Disconnect one of the power sources from wherever it comes in, and adjust the 3-way wiring as needed to work the correct way (with a single power source – one of the switches only gets power from the other switch, IIRC. It’s been a while so I’d have to google a correct wiring diagram.)
I don’t know code, but I’d bet dollars to donuts that it’s against code to tie two circuits together, since that defeats the purpose of the circuit breakers. You could have twice the rated current in the wiring before tripping either breaker. (Once one trips, the other would too.)