I had just completed a few minor repairs to a client's tenor sax, and after I'd played it and satisfied myself that it was working properly I handed it back to the client to try out. He reached into his case and pulled out a bright steel object that couldn't have been anything other than a Berg Larsen mouthpiece.

He blew a few scales on the horn, and I knew right away that something wasn't quite right.
I know the sound of a Berg - or rather I know what it ought to sound like when I see one being played. The middle spread of notes were fine, or at least from low G up to the octave. The cut of the tone, coupled with a relatively flat response made me guess at the mouthpiece being a 95/2 - but this guy's tone fell apart outside that single octave. More than that, the notes fell apart too.
He was by no means an accomplished player, but was at least good enough not to have had such problems with his embouchure.

He stopped playing and looked at the horn with the sort of concern that comes when you're facing the dilemma of handing over hard cash for a job that's perhaps not all that well done.
I jumped straight in - "Is that a 95/2?" I asked. "Yes", he said, "how did you know that?"
With perhaps more nonchalance than was decent I simply said that it had that sort of ring to it....and only just resisted the temptation to say "well, when you've been in the business as long as I have..." Truth be told, it's not that difficult - it was a bright mouthpiece, so it could have been anything from an 85 to a 100, and it had just enough depth to put it closer to 100 than 85. As for the 2 ( which equates to the tone chamber, running from 0 to 4 ), there was a one in five chance of getting it right - and as you see very few 3's and 4's that knocks the odds down even more.
OK, OK, I made a lucky guess!

I asked how long he'd had the mouthpiece, and it turned out he'd bought it secondhand only recently. I took it off him and examined it - and confirmed my suspicions. Someone had 'worked on' the mouthpiece.
I put the term in quotes because there's a whole, big, fat, wide world of difference between carefully tweaking and honing a mouthpiece to enhance its every nuance and simply taking a file to it. This piece had met the latter technique.

Under the reed the rails were at least twice the width they ought to have been, and the tip was easily three times its normal width. Heaven knows what amount had been taken off the piece to do that kind of damage. No wonder the mouthpiece didn't work at the top and bottom of the horn - it's a wonder it produced any kind of sound at all!
I had to break the news to the client - who was understandably miffed at having shelled out cash for what was effectively a paperweight, but then again it has to be said that if you go buying mouthpieces without the benefit of some playing experience under your belt you stand a good chance of buying the wrong one...or, as in this case, a 'duff-un'.