I've done it once - my mind just happened to slip at one sale, and it didn't register that I sent the wrong one until the customer convo'd me here. She was completely understanding though - I suggested she keep the buttons I sent her and I'd send her the right buttons free of charge. :)
My system is to keep the items, invoices, etc. together in a line, and far away enough from each other that they don't get mixed up. And, of course - double, triple check! :D