Intonating a floyd. You get a feel for how much the saddles have to move after a while, but till you have it dialled in its a PAIN.
After that my least favourite was correcting for an incorrectly set in neck (bowedback at the heel so that 22nd fret was about 0.5mm higher than 12th) to get the action that the customer wanted by taking the frets down (after explaining to him that this is a non-optimal way to correct the problem, and the alternatives, its what he wanted)
One after the other
Each lower than the last
By exactly the right about (1/20th of a mm) to get the surfaces in correct line with the strings.
Hated that.
Oh, it was on an Epi Zakk wylde. Avoid. Or do not purchase without thorough inspection.