Throughout my years in the web industry, the subject of SEO has become very controversial. SEO has also seen many changes and phases since it was first introduced. While SEO practices are every changing as the Search Engines become smarter and smarter, the following are a list of some SEO Sins that will never become main stream practice again...
1. Hidden Text: the practice of making content on your site, match the color of the sites background is considered a no-no. Black hat SEO companies may use this technique to try and increase a pages keyword density by creating hidden text that just contains a lot of keywords.
2. Doorway Pages: these are pages that are specifically designed for search engines to rank highly for one keyword phrase and then will redirect a user to a different page when they visit, commonly known as "bait and switch". Pages like these commonly use the keyword phrase repetitiously and try and trick search engines into ranking them well for that particular keyword phrase.
3. Doorway Domains: these are multiple domains with duplicate content. This technique uses URL redirects that are meant to display another web address for the same web page or several domains show same content. Black hat SEO companies will use this technique in hopes that they will be listed on a Search Engine Results Page for at least one or all of the URLs.
4. Duplicate Content: going along with #3, many site owners try to increase their content base by creating multiple pages of the same content. They will either make multiple pages within the same site, or copy the site over to additional domains, so that they exact same site is out there multiple times.
5. Cloaking: this is a technique in which a site serves keyword-stuffed spam pages to search engines spiders through the detection of their IP, while they deliver completely different pages to their customer.
6. Keyword Spam: this is a technique in which you attempt to overload your pages with keywords. Stuffing keywords into the Title Tag, Meta tags, Anchor texts, and Alt Attribute tags in an attempt to increase your keyword density or to increase the amount of keyword phrases you can target per page, is considered spam. For best results, you should focus your pages to contain two or three keyword phrases.
7. Frame-based Websites: Older sites were initially built using frames. If your site is built in frames, there is a very real possibility that your sites content is not accessible to search engine spiders. For best results, you should re-structure your site using Cascading Style Sheets (CSS).
While Frame-based websites will not get your site banned, they will prevent your site from being indexed properly by search engine spiders. However, point 1 through 6 are major SEO no-no's and can potentially get your site black listed from every search engine out there. If you are caught and your site is black listed, you will have to go through the long process of removing all black hat SEO tricks, and then re-submitting your site to all search engines for them to review and confirm that you are practicing proper SEO techniques. Only then will they even consider allowing your site back within their index.
At the end of the day a search engines main goal is to provide users with the most reliable, accurate information regarding the searches they perform. Don't start off on the wrong foot by trying to trick them.