Actually, errors in old casino matches are pretty rare. I've seen many more error chips than error matches. I went through my duplicate Silver Slipper covers and didn't find any missing "Nevada" as Mark showed earlier.

There is sloppy workmanship on the covers quite a bit, but I don't consider slightly mis-registered colors or smudged ink to be an error. I guess, there's less incentive to inspect covers for these workmanship problems than there is for chips which have a money value.