I’ve got a switch in my flat with a post-it note saying “do not flip this switch”. I’ve been here nearly 6 years and still haven’t.
We have two in the living room. They controlled an overhead light that had been removed long before we moved in.
I almost created one in our previous house. I took out a half wall and replaced it with stair rail. There was a switched outlet on that wall. I was about to put the base plate for the rail down when I remembered the outlet (the capped wires in the framing below were a pretty good clue), and I had already painted the wall with the switch so I didn’t want to cover it up. I found a new location for the outlet.
Some friends in CA had a mystery switch in the kitchen. There was a large window in the kitchen different from all the other windows, an exhaust fan was probably removed when that window was put in.
There is a phantom switch in my living room. That is the only room in the original house that doesn’t have an overheaed light, so I assume it once did. If I am ever tempted to put in an overhead fan in the LR, I assume the wiring is already there.
When we remodeled the kitchen, we had the contractor use an existing circuit (previously a light) for the new dishwasher and the electric ignition for the gas stove. I know what it does, but it would confuse most folks.
I have several, but as the house was built for us I’m not sure they count as phantoms.
9 switches for ceiling fans that are unused because the fans we installed have radio remotes, so only one switch is needed.
One more for track lights that are not yet installed.
Back in the days when TV channels were received via an antenna, you could buy a signal booster to improve the picture quality. Well, ours was wired to our deck lights. So, to watch television inside, you had to turn on the deck lights outside. Very handy.
We have GFIC outlets in our garage. Rather than have them reset in the garage, or in the bathroom nearest the garage, we have the reset on the other side of the house in the kitchen. Because, that’s the logical place for it.
There is a switch right inside my front door. It’s not really a phantom switch, since I know what it does, but the wiring is so jacked that it was a phantom switch for over a year after I moved in. It turns on the landing/main hall light. But if you turn it off using that switch (rather than the one in the main hall), you have to go up a flight of stairs to the main switch and flip that one to the off position, come back down and flip the “phantom” switch back on, then go back up and either turn the light back on using the hall switch or just say “Fuck it, I don’t really need that light anyway”.
The house I lived in before this one had a similar wiring problem: the overhead living room lights had switches in the hallway, kitchen, bathroom, my bedroom and the living room proper. 5 switches for one light fixture. They all worked, but if you turned the light off from either my bedroom or the bathroom, it could only be turned back on using that particular switch.
Often builders will install an extra switch for a ceiling fan that is never installed. Many times there’s an extra wire inside the box for the ceiling fixture for that purpose. That and switched outlets explain a lot of phantom switches.
We have a double switch at the top and bottom of the upstairs stairway. Half of each does nothing. At one time, there was probably a ceiling light at the bottom of the stairs, long ago removed. Also, half a double switch in the back hall, once for an outside light.
We have a panel of five switches right inside our front door. Two of them controlled outside lights we’ve since put on timers. Those are now phantoms. Two more control a porch light and the hall light. The fifth – who knows it’s always been a phantom.
We have a couple in the mud room and living rooms. Annoyingly, we have motion sensitive flood lights in the back yard that have the opposite problem: they’re not associated with any switch I can find so there is no way to switch them off outside of removing the bulbs.
There are a few on my house. Three on one panel of 6 in the kitchen. They were for lights that were outside, but were removed when the addition was put on the house.
In the rental I have a switch inside a room which didn’t seem to do anything. Then at one point the light in that room wasn’t working, and it turned out that in that room if the lights are off, you have to use the same switch you used last time. If you use the incorrect switch and then use the correct one without re-switching the incorrect one, it also doesn’t work.
My opinion about that electrician is not very polite.
In my own house there are two switches in the lights panel that have identical consequences but are officially different (both of them shut off the electricity completely). In principle you’re supposed to use the higher one because it’s the one connected further out.
The only reason I clicked on the Thread. 
I once checked into a hotel in London. When I walked into the room, I saw that a lamp was on. When I wanted to sleep, I tried to find the switch. There wasn’t one. I looked & looked. Finally, I phoned the front desk. He seemed very confused, “Why, no, sir, of course, you can not turn off that lamp. It’s on continuously for your convenience.”
“I want to sleep now”
“OK, good night, sir” (click)
I thought about unplugging the thing, of course, but the outlet was unlike any other I have seen. I would’ve needed tools and time.
Ultimately, I removed the light bulb, for my convenience, and threw it into the bin.
Yes, I installed it. When I finished off my basement into a rec room I had planned to add some tract lighting on a section of the ceiling where I had to drop it down to 6.5’ to cover an air duct.
It was a double switch with one switch for an overhead light in the center of the room and one for my tract light. All too late I discovered that the tract light would hang too low so I removed the cable for that half of the switch and left a “dead” switch in the switch plate.
In the house before my present one, there was a switch beside the back door that appeared to have no purpose. Almost a year after we moved in, we had some of our neighbors over for a picnic and one brought a boombox he wanted to plug in. He reached behind a bush and flipped up an outlet cover we had no idea was there and plugged in the boombox. When it didn’t power up, he said “go flip that switch inside the back door”.
So it appears the phantom switch was only a phantom to us, the newbies.
Have a switch by the front door that obviously used to control the porch light but at some point in the past the apartment folks decided to make those lights automatic.
I created some in my house. Originally, there were 2 switches for the light fixtures in the kitchen and living room (2 for each room, I mean). Then there were 2 or 3 other switches that, if not configured correctly, would shut off the wall circuit in the living room. I had to draw arrows on the switch plates in order to make sure they were flipped the right way, or the tv wouldn’t work.
When I had to replace some switches, I disconnected the troublebmakers.