What can get you penalized?

Keyword placement, keyword proximity, keyword density is all important factor in a website development.But overdoing those like placing keyword on every part of your page will do no good for you.This is termed as Keyword stuffing by experts and in search engine terms they call it as keyword Spamming.

The consequences is that your website may get penalized by ranking it low in SERP.

Don't make your page heavy,mean to say one should not overdo with the images just like mentioned below:
No heavy Images.
No Heavy Flash
No links in Flash

Another factor is Cloaking which by far is an important issue as far as penalization is concerned.

Cloaking is an unethical SEO tactic which involves serving a specific page to each search engine spider and a different one to human visitors.

There are three types of cloaking,
1)Ip based
2)User-agent based and
3)Hybrid

Ip based cloaking involves showing different pages(which are highly optimized with fresh content) to the search engine based on their search engine(IP).

User-agent based means showing pages according to the type of user like mozilla(which is an user-agent).

Hybrid ia an mixture of the both IP and User-agent.

IP based cloaking is by far used more to serve the cloaked pages to SE.