I paid a gas plumber full rates to in effect remind me where the tap is for the fill line for my boiler.
Over time a boiler that isn't meaningfully "leaking" will still gradually lose internal pressure. Once the pressure inside the boiler is less than outside the boiler even when the water inside is hotter, that's not good. Eventually the system won't work and shuts down for safety, but before that it'll make a lot of noise while running. There's a pressure meter so you can see what's wrong if you don't understand boilers. My meter said about 0.4 bar. So that's too low, now I just needed to re-pressurize it.
The regulations here say that, to avoid mistakes resulting in stagnant water flowing back from a boiler tank into the fresh water system, the two mustn't be permanently connected. So there'd logically be an input, you connect a temporary hose, re-pressurize.
In practice, nobody does that, the installers will run a permanent line, in defiance of the rules, and use a tap, now you can re-pressurize by turning the tap.
Except, many years after moving in and setting up I'd forgotten where that tap could be. It wasn't where I expected to find it, and I could not think there else they'd have put it. So after sleeping on it and still not remembering I called a plumber. Not an emergency plumber, but still plumbers aren't cheap.
The plumber also couldn't immediately find it, it wasn't where he first looked either. But I can't actually tell how much time he spent "pretending" to look versus how long it really took him to discover it. Because it's embarrassing right? Even if the customer is up-front about the nature of the task, "It's under the kitchen sink - behind this panel, here" (yes, that's where it actually is) doesn't feel like a real job worth £80 or whatever it was.