Mine are:

a) I could not find any spanking content that I personally liked. So In the start, I had to make it for myself. It was only later that I found that you could make money off of them...and I was rather suprised. I never started out to be in the Adult Industry...it just sort of happened. Once I was selling my personal tapes (back in the VHS days) and making some extra cash...things went forward. One day I realized I was making more at my hobby than I was at my real job. Goodbye real job. I still have a lot to learn, and I am always inspired by things along the way.

b) Trusting someone in this business that I should not have almost cost me everything. I am still recovering in some ways...not the least emotionally. I have met a lot of great guys (and gals) in this industry...but there are also some sleazebags. So I guess I would have to be a cynic and say trust. Although, now I find that resources like this board, the forums, internext, etc. give me an opportunity to meet people, interact, etc. and I feel like I have a network of trusted friends now that I can lean on for advice. So maybe in the long run, that one mistake ended up being the most profitable.
Oh yeah, and Google Adwords never really panned out for me either.