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In other words the bad guy may be sneaky and wear a white hat instead of his black hat.
1) Phony links traded for for real (<a> href=) links.
2) Scraped / plagiarized and/or uncredited content
3) Blatant KW stuffing, especially when cloaked or masked.
4) Link farms, link rings/triangles and variations.
5) Framing the contents of other sites without permission or notification.
6) Click attacks on competitors adsense or other ads.
7) Denial of Service attacks (DOS)
8) Rewriting pages of other sites to capture them within the BH domain.
9) 301 page hi-jacking.
10) Machine Spamming blogs or guestbooks to gain links.
That's only 11 things off the top of my head. There must be many many more. -Larry