Well, 4 years ago, long before we had Apple stores here, we did have an issue with 15 MacPros. The dealer put them up, everything was fine but ALL of the 27" Displays had problems. Green corners, purple corners, lines, pixes errors, subpixel errors, you name it, each and every one of the 30 monitors had something wrong. Mind you, brand new. Some got changed right away, only to arrive with more errors.finally we had 12 working and flawless monitors, and on the other 18 where at least one, mostly more pixel errors. According to the supplier that was "normal" we did not think so. But the supplier came with documents that said, that up to 2 pixel errors or 4subpixel errors had to be accepted.
We then wrote (a letter, not email) to Apple, Infinity drive in Cupertino CA.
With pictures, a detailed account of what happened and a copy of the bill, after all only for this installation over 62000 Euros.
We waited 2 weeks and where working with the machines (we are still working with them) when a VERY friendly and apolegising guy from the system house came, with 18 flawless 27" monitors, free of charge (he left the 18 monitors we had with the errors) a letter from Apple, excusing themselfs and as a token a 15" MBP, also free of charge.
Granted, we do spend 6 figures with Apple but I still think that is good service.