Search engine penalties are reserved for pages and sites that are purposely attempting to trick the search engines in one form or another. Penalties can be meted out algorithmically when obvious deceptions exist on a page, or they can be personally handed out by a search engineer who discovers an infraction through spam reports and other means. To many people's surprise, penalties rarely happen to the average website. Most that receive a penalty know exactly what they did to deserve it.

* Duplicate Content: This filter doesn't matter for much of anything. Only one copy of a syndicated article should rank in the search results. If they don't rank all of them who cares? Even though duplicate pages are filtered out of the search results after the search query, they still pass link authority, so the idea of remixing articles to pass link authority is a marketing scam.

* Reciprocal Linking: Natural quality nepotistic links are not bad (as they are part of a natural community) but exclusively relying on them, or letting them comprise most of your link authority is an unnatural pattern. A friend's site that was in a poor community had their rankings sharply increase after we removed their reciprocal link page.

* Limited Authority & Noise: A site which has most of it's pages in the supplemental results can bring many of them out of the supplemental results by ensuring the page level content is unique, preventing low value pages from getting indexed, and building link authority.

* Over-Optimization Filter: I had 2 pages on a site ranking for 2 commercially viable 2 word phrases. Both of them were linked to sitewide using a single word that was part of the two word phrases. Being aggressive, I switched both sitewide links to using the exact phrases in the internal anchor text. One of the pages now ranks #1 in Google, while the other page got filtered. I will leave the #1 ranking page as is, but for the other page I changed the internal anchor text to something that does not exactly match the keyword phrase. After Google re-caches the site, the filtered page will pop back to ranking near the top of the results.

* many forum members are complaining about similar sites or page types getting filtered or penalized (although it is tricky to find signal amongst the noise)

* reading blogs and talking to friends about algorithm updates (much better signal to noise ratio)

* seeing pages or sites similar to yours that were ranking in the search results that also had their rankings drop

* knowing that you just did something aggressive that may make the page too well aligned with a keyword

* seeing an inferior page on your site ranking while the more authoritative page from the same site is nowhere