Google recently introduced a new keyword phrase filter during its most recent update. Some phrases were unchanged, but many highly optimized and highly competitive phrases were drastically altered. Some webmasters saw their sites drop from top listings to not being in the top 1,000 sites. Unlike other filters, this spam peanalty does not affect a page or sites overall perceived value. The end effect is lowered rankings for various specific searches for that page.
If you have a couple commonly paired words on your website such as "search" and "marketing", the new filter will check to see how the words are placed on your page (and possibly in incoming links). The new algorithm is looking for more naturally existing context to prevent search engine optimized websites from dominating search results. Essentially the goal of this filter is to get webmasters to adjust their content to be written for the users and not search engines. The problem with this logic is that good SEO typically falls in line with good site and page structure.
The filter is selective in that it is only enabled for some terms on certain searches. The filter can be overridden by placing a - character for the terms. If you search for search marketing you will find Commision Junction where my site was once listed. If you search for search-marketing you will see my site. If you search for search marketing info, you will see that the filter in not enabled for that search.
Off the start the filter was set rather corse, but it appears to be shifting more in line with reality. Some of the decent optimized sites are appearing back in the place where spam was just filling the results. Just this morning I saw a clients website pop up for two seperate phrases he was tripped for.
This filter marks the first time in search engine history where webmasters have not been able to reengineer exactly what the search engine did. Currently I do not recommend making any chages while Google is still fine tuning the filter.