The big one is "That no-one would buy Magic: The Gathering."

I can't find it on google groups, but there was a post way back on rec.games.design (or r.g.d.board) from someone called Richard who said something along the lines of 'I have this card game that people seem to like and we're thinking of selling if we can raise the money. You both have a subset of cards from a very large deck – which you don't have all of and will have to buy in small pieces, like trading cards – and you don't know what the other has, so there are plenty of surprises. Oh, and if you lose, you have to give them some of your cards.'

I thought, "No-one's going to want to play that, never mind buy it. Where's the fun in being trumped by a super-whizzo card you've never even heard of? Where's the skill element? You can't be sure of getting any particular card – you can bet they're not going to let you pick which you get in the packs. And after splashing all that money, you have to give the cards away?? Nah."

Several billion card sales later…

… I'll confess I still can't see the appeal. There are lots of better card games out there. But I was wrong. And had I put up the some of the money he was looking for…

Oh, another belief is that "People will not pay large sums of money to have a crap website."