A while ago a friend of mine pulled an interesting card in a pack of Urza's Legacy. The back was normal, but the front was solid black with a white M on it. I can't find the card right now, but I made an image of roughly what it looked like.
I think at the time I was told it was a placeholder for when there was extra room on the cutting sheet, and was accidentally included in the pack. I cannot remember which rarity slot it occupied.
I am trying to find out how much it is worth, or if there is any information about it online? Googling "magic m placeholder" and variants of that are not turning up anything useful.