Unusual back linking
A SEO company may put your site in 10,000 directories within a month. By this way they will help you to get a very short time high page rank for your site. But it will also ensure the death of your site. Cause Google has the ability to understand how you have managed this kind of back linking in this short period of time.
You selected certain keywords for your web page. And after that you used those keywords in a meaningless way in your web site content. Each and every line you are using those keywords not keeping the thing in your mind that your site content is for the user not for the search engine boat. Search engine like 3%-7% density for keywords in a content page. But if you make the density of keywords more than that they will send you in their black list.
Search engine like contents a lot contents in a web page. But sometimes too many content is off for site layout. Developer tries to do some tricks. They use 1 pixel font size with a colour very similar to their web page back ground. It will keep all the contents into the web page but those are hidden to the user. Developers must have to know that GOOGLE is smart. They already know these things.