I'll start it off: Mistake: Focusing my site on making money and not providing for the visitors. Success: Trying out word-of-mouth promotion. I hope you all get the idea!
mistake: just creating content without testing keywords for popularity. succes: testing what keywords do work and building on that.
This is for my website design business: - Mistake: not being clear enough with quotes (so I end up working more for the client than initially) - Success: doing a good job an receiving returning clients AND word of mouth advertisement