We've mentioned numerous times on the site that cop black was accidentally left out of Alpha.
But another card was also inadvertently not printed in the premier set: Volcanic Island. Because dual lands were treated with contempt in the old days ("A land as my rare?! RARRRRRGH!"), and very few people owned entire sets, its possible that no one even noticed that only nine of the ten two-color combinations were accounted for. In any event, the land was slipped into Beta and they all lived happily ever after.