That's not as bad as Cuenca, who didn't know the name of the black mox and ended up registering an Opal instead of a Jet in his BUG list (yet still top 8ed, because Nicolas Cucino doesn't lose matches of magic)
Oh man, that was actually hilarious. I was getting blown out by Nic with his BUG fish deck, and he drops this nice looking acetone-d cleaned proxy with "mox opal" written on top and "tap for B" written in the text box. He goes to tap it and I'm like "you don't have metalcraft."
We all thought it was pretty funny, cause, you know, he was already winning that match, whether or not he was playing with a mox jet, mox opal, or a Baron Sengir.