missing valve shim

[White Anthony <anthony.white@domain.elided>]

And something really weird:  When I took the valve followers off to check the size of my current shims, I was missing one.  Completely.  It just wasn't there.  The end of the valve looked like it had been gently hammered with a ball peen hammer for what I presume must have been 50k miles, and the valve follower had a stress pattern on it that looked like a snow-flake.  
 
What I don't understand is, why had it not been making some god-awful noise until I pulled the head to replace the head gasket?  It's been running like a dream since I bought it six months ago.  
 
I pulled the head off intact, with the cams in place.  The valve clearances were all within spec.  I put the head back on, and the valve clearances were still within spec.  I took it for a drive, and after a couple of minutes, it started making a terrifying racket.  I said things that aren't repeatable on a moderated list.  I took the cam cover off, and there was obviously a clearance problem with the exhaust on no.3 -- 1.7mm clearance.  I put a spare shim in, and the noise reduced to the normal slight valve-clearances-slightly out noise.  I've ordered a new follower to replace the one with the stress pattern, and will set the clearances properly when it arrives.  
 
The previous owner was a pure chequebook mechanic -- I have receipts from him even for oil changes and new sparkplugs.  So I gather it must have been the Alfa shop that messed it up.  It's the fact that it wasn't apparent until now that confuses me.  
 
Regards,
Anthony.  
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