I don't think Band of Misfits was a mistake. Well that sentence is wrong, I didn't have bacon for breakfast today, and that was a mistake, and I'd be very mad at DXV if he proffered an opinion on whether it was. But what I mean to say is even if it's a mistake for DXV's goals, the card was really exciting for me when I first saw it, and fun to play with in the games I played, Dark Ages being one of very few paper sets I own. "This is that card" was such silly fun. "This plays some of other card" wouldn't have given me the absurd abstract art joy and may have required more explaining to my family.
We can't run the experiment to see what joy the errata'd version would have brought, but I wasn't saying it was a mistake to not already have the errata in the first version (though I sure bet it would have produced about the same amount of joy; it's still the card that can do all those other things the cards on the table can do). The mistake was doing the card. I could have waited years on it, made a version of the concept when I had one that worked. That slot could have just been some other card. As always, it's not "Band of Misfits vs. nothing," it's "Band of Misfits vs. whatever else." Whatever else might have been great.
Band of Misfits left Dark Ages for having rules problems. I put it back in thinking, what, it's fun, how many more expansions will there be. A bunch more, it turns out. The rules problems cause endless questions and confusing situations. It's great if people like the card; the move was to not do it. Just before Dark Ages came out, there was no Band of Misfits, and people didn't say, man why do I even play this game, not enough excitement, where's my shapeshifting card.
I continually make cards that try to break the rules in whatever new way, risking problems. I do this because maybe you get something cool. And it can work out to just having good times; I don't regret all those cards. But shapeshifters make the game rules not work, create undefinable behavior. There are different approaches possible to fixing them (e.g., not letting you play a card if you can't put it into play kills majiponi's problem situation, though it also kills e.g. throning one-shots), but with piles of expansions, the move was to change as little as possible.