One real world example, about "fabled tone" .
One Music Electronics Forum member, John Frondelli, runs a mythical New York Service Shop, where *all*and I mean *ALL* stars, nopt only Rock but everybody else (including Disney Studios, all NY Studios, Oscar organizers .... up to and including the Navy band and CIA

) send their stuff to be repaired, every month a truck stops at their door and blocks the street with stuff from, say, Megadeath or Prince or whomever is going on World Tour and wants everything checked for safety, nobody wants to rush find a Tech 8000 miles away from home.
One day he said U2 had left
their truckload of equipment, and everybody asked him about a fabled Fender Princeton which allegedly had been used to record all famous band Hits.
Intrigued he checked it, since in his eyes it was "
just a normal Princeton, bought at a regular store and never modded" yet it was labelled with Sharpie on a piece of paper strip: "
repair but do not modify".
He checked it part by part, schematic in hand, ticking one by one what he found "normal", all he could find was that it was exactly the same as all others (at least nominally) with the single exception that a .022 coupling cap had been replaced by a .01 one.
Not a Mod, it was there from Day 1 , put straight at the Factory.
Net effect was that it cut a little Bass, so when overdriven, distortion was
slightly cleaner than all others; in any case a subtle effect, not day and night.
In another Tech Forum, a member used to be a Fender VP for some time, and was asked about it, "
what were they experimenting or trying to achieve and why weren´t all other Princetons built the same way, given it "worked better" His answer? : "
don´t read too much into it, probably that day they were out of .022 caps, they *had* to ship out some of them, and all they had on hand was .01 caps"

Since apparent effect was very small and still acceptable, so be it.
This matching what Enzo said about amp to amp variations, in this case because of an actual value change, not just tolerance spread.
Clearly The Edge tried a couple Princetons at some shop and bough the one he liked most

Just to check the *awesome* customer list who relie on that particular shop:
http://www.dbmproaudio.com/customers.html