I don't think I've ever been truly disappointed with a lens...
I've had a couple where more than one brand new copy had issues - the DA16-85 in particular - and that resulted in me returning them and choosing something else (in the case of the DA16-85, I really wish it had worked out differently - but no biggie). I've also had a couple that required alignment before they performed properly, but that happens. In most cases, though, I've only found some unexpected limitations or characteristics which I've learned to work with - and then I've been happy with the results.
I have had a couple of vintage M42 and Adaptall eBay purchases that turned out to be lemons, but they were returned and refunded - and I've missed a couple of great bargains over the years, due to procrastination or losing track of time!
As a side note, I'm surprised at how many find the FA50's so disappointing. I bought a used f/1.4 earlier this year and I really like it. Wide open, it does have a particular signature look... lacking a bit of contrast in certain situations (I do find it benefits greatly from a hood at wider apertures) and out-of-focus areas (highlights in particular) can be a little busy, but it has plenty of character. AF fine adjustment needs to be spot on for sharp results wide open, of course. Anyway, I guess it just goes to show how tastes can differ