I have yet to hear of misprints or player mistakes made due to fan made cards.
I always have the official versions with me so when someone is bothered by it, I can swap them out. It would be no big deal if they did ask that. I've never had someone complain about fan cards though. Usually they say nothing because they know what the card does already or they compliment it and/or ask where I got it from.
I see fan alt card that players win at events like Stahleck, as a non-issue.
We can agree to disagree here on whether it adds more fun to the game or not. I personally don't like it, but I understand that you guys find it more fun.
Just need to point out that I did refer to the first page of this thread, where Mel's strength was shown as 3. it is 4.
So the anecdotal evidence, such as "I have never seen", is usually poor argument.