As for why doing the project again would have been a mistake, asking people for consent is not a cost-free thing, and many such events work far better with fewer participants for reasons both obvious and subtle.

The real mistake theferret made was posting about this on the internet. I was involved in a discussion about the OSBP on the xkcd forums when the post happened, and was amazed by the degree of misunderstanding and overreaction among people condemning it. That was the sort of reaction theferret should have seen coming, and kept the project an invite-by-referral thing rather than a public recruiting thing.

The event as proposed did not control sufficiently for "consenting". (Or "adult", for that matter.) That was the exact problem, in fact.