OK you're smart and have huge SEO successes under your belt. But chances are, you may be either too good or just plain lucky. Your strategies may have worked out so far, but be careful and check whether they are fool proof. You can't take Google for a ride. You may have got away by fooling most of the people most of the time, but remember, you can't fool all the people all the time. Google is just too smart and you may have got away with some minor irregularities somehow, but be careful or else you may earn yourself a ban.
While the following three errors may not earn you a penalty or a ban they will definitely affect your ranking:
Hiding Text It will definitely earn you a lower ranking. It may be something like colored text on a background of the same color. If you use JavaScript and don't display the content or use CSS it is also considered undesirable. You may invite a penalty.
Cloaking If you display one page to a search engine and a different one to a user it is also considered unethical and may also get you a ban in the process.
Flash Navigation This may block the search engine crawlers from viewing your site. This will naturally result in lower rankings for your site.
You would do well to avoid the following:
Overloading keywords
Very generously spraying keywords around is a SEO tool, but using them very often just for the sake of getting a better ranking is considered 'unethical'. Search engines like Google dislike it. So keep the keywords to the optimum level.
Copied Content
If you use content that is copied or seems very much inspired by from another site the search engines will be quick to spot it. Also do not repeat the content on subsequent pages. Some sites are also protected by copyrights and have software to prevent any copying or trace any copying jobs. You may also get a ban.

Unrelated Meta Keywords
You notify search engines about keywords by using Meta keywords. Don't use irrelevant Meta keywords that are popular just to pull a fast one on the search engine. You will ultimately be caught and it may carry a penalty.
Link Farms
Don't build pages in your site just for the sake of increasing the number of links to your page. If the number of links in a page exceeds 100, it is considered a link farm. Just increasing the number of links to a page will not benefit you in any way and may also attract a penalty.
So, although you may get search engine optimization in the short run by using the above techniques, but it may cost you dear in the form of a penalty or a ban.