A Webmaster Blog
Google has some way of isolating certain keyword phrases that they deem typical of spam sites.
But the number of keyword phrases that can cause the penalty depends on other factors like
1. Indexing
2. Weighting
3. Ranking
4. Duplicate content
5. Spam detection and weighting (yes – not all spam is bad?)
6. Back Links/Link Profiles
7. Personalized Search – [...]