I do this quite frequently...I just don't hesitate to get things done, ever. At least you might see it the first time and change it immediately after. Me, I spend the whole duel thinking, "Gee...I'm pretty slow today..." then halfway through the duel notice I have a random source of fire/earth quantum emanating and look at my mark to notice my blunder. And the saddest thing about this? Most of the time I have this happen and forget to change it again immediately afterwards...can you say FAIL?