How about the schrader valve wrench - the thing that removes the valve in the air stem of your tires.
The water meters for houses in some cities have an unusual connector that the average Joe can’t access. It also means that a plumber installing a “rough”* can’t water-test the system.
*A Plumbing Rough is installing drain pipes (along with pipes to “catch” things like commodes, showers, tubs, etc) in the dirt before the foundation is poured. In order to test the system for leaks, plumbers will seal the outlet and fill the system with water. If they get the water from a meter box, the city considers it theft.
Hell, if you have a Dremel with cutoff wheels and grinders, you can make one of those in about ten minutes from a small - say 1/8" - hex nut driver.
true but it does answer the OP
I have machined custom wrenches for fire suppression systems. While it is not a common tool - actually non other like it, it is not made to prevent somebody just walking by from messing with it
For some unknown reason, certain McDonald’s Happy Meal toys are put together with screws with a triangular-slot head. Again, it’s trivial to make the tool with a Dremel grinder, but I have to wonder why they felt the need to use such an odd screw.
Actually, they’re to keep away the tigers… 
[hijack]
While we are on the subject of Dremels and their capabilities, I thought I’d take this opportunity to brag about how I created from (almost) raw materials a copper retaining clip for a shower cartridge. The water had frozen and forced the cartridge out of the housing, destroying the old clip. I was unable to find a new clip without buying a new $150 valve. So I took a 1 1/4" copper coupler, cut it in half, hammered it flat, and proceeded to use my dremel to carve out a new clip . I rule.
[/hijack]
Just so I can say something of relevance to the thread, I also have taken an old metal handle from something or other, cut it off, and remade the tip to serve as a tool for installing and removing a Loop-Loc pool cover.
And all without the use of duct tape!
Lots of movie theater seats use the same screws, for the same reason. 
[obvious straight line]
I’ve never seen a tiger in a public restroom!
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See how well it works?
(not previewing to reduce the chance of someone else saying this)
That’s only because Hobbes didn’t need to pee while you were in there also, QED.
Hey, it’s not Q.E.D. but Qadgop who’s having problems with Calvinists!
Holy Cow! This is exactly what I was prepared to write. I cut my teeth on 1960’s Mustangs, it was always a running joke with the guys about how you needed a special “Ford tool #3769a” to fix something.
Say…your not from Missouri…are you?
Maybe the company was just using up screws left over from another production run.
I’ve removed and re-used any number of those clamps. Just loosen the crimp with needlenose pliers and/or a hefty pick or screwdriver, and you get enough slack to undo them. Once they’re apart, it’s not hard to reshape the crimp area so as to be able to recrimp it. You get a nicer crimp when installing them if you use the right tool, but you can certainly get a functional crimp with the clever application of the right sorts of pliers.
Ahem. And you have the special drill bit/socket because . . .?
[quote=PoorYorick]
Ahem. And you have the special drill bit/socket because . . .?
[/quote
Because steel security doors use the same screws.
Thanks, CynicalGabe.
All the restrooms where I used to work had those great big, huge-roll toilet paper dispensers. It took a big plastic key to get in, but the cheap latch was balky, so the janitors had ripped the covers off every one.
The one-way screws in the stall walls there had shallow, rounded heads. Apparently, some guys amused themselves by backing out the “tamper proof” screws with Channellock® arc joint pliers.
I came across an '82 Chrysler once with a power steering hose that required a 23/64" wrench to remove the fitting. And no, that doesn’t translate directly to any common metric size…9 mm is too small and 10mm is too big. The car was built in Canada but everything on it used regular inch measured (imperial?) wrenches.