I tested a few of my sites to see how Google would respond to adding a long list of keywords in the meta tags and on the page itself (a different color than the background and the same color as the background). This was just done for educational purposes to see what the engine responds to 
.
Anyways these are the results that I got:
1. Spammed keywords in meta - did not drop my sites very much if at all in Google, but didn’t help bring in more traffic via search results.
2. Spammed keywords on the page but visible - this caused the rankings on my sites to drop a couple hundred ranks, but it popped back up a couple of days after I removed them.
3. Spammed keywords that are the same color as the background (invisible) - when doing this my sites completely disappeared from Google. This really pissed off the search engine. After removing these from my test sites, the sites reappeared in Google within 3 days.
Seems Google updates very frequently and quickly so people can lose their top rankings in a snap.
blog walking