Bizarre Monster Book Error

ok, but here's the dilemma with that: If it's displaying Leviathan as having 5 cards, and then I get a Leviathan card (even though I don't really have 5 leviathan), what happens?

I wouldn't know. My guess (according to what I said above) would be that it adds 1 to wherever Leviathan got shifted too. Or, it thinks the card is whatever got shifted to Leviathan (the 5/5) and doesn't add it. Personally I wouldn't try to find out.