Just picked up a Sapphire Nitro+ 580 4GB, and after BIOS modding it was kicking out incorrect shares constantly. I went back to the stock BIOS and with a modest memory overclock it's still getting one every 5 minutes or so while also getting some accepted shares.
It's got Hynix memory and 78% ASIC quality. The rig is three 480s, a 570, and this 580 mining on nanopool, and it's very stable. None of the other cards get incorrect shares that I've ever seen.
Is this card simply a lemon?