There are seo techniques that can get your site banned from Google if you're caught or one of your competitors notices and turns you in for spamming. These techniques, commonly referred to as "black hat" techniques, are the topic of many forum threads, and have brought ruin to many an unsuspecting optimizer.
Optimizers who use so called black hat techniques regularly have methods in place for managing search engine banishment, such as many, many registered domains.
Ammon Johns, in a thread over at Cre8site, removes the black hat vs. white hat seo distinction from the realm of morality and places it into more pragmatic terms: risk. (I also have to credit him for my title - his quote really got me started: "there are no hats.")
Throughout the thread I read repeated warnings against using high risk seo techniques - these are tactics that, especially if you're new to seo, you should not use, as you will not understand their actual risk and may end up causing severe damage to your business and livelihood. Beware.
Here's part of the thread that caught my attention, and the attention of the SEORoundTable blog, where I found Mr. Johns' concept:
There are three forms of SEO tactics:
1. Techniques currently rated as 'safe'.
2. Techniques rated as risky.
3. Risk balanced and risk managed techniques.
In WebProNews we focus on techniques that are considered safe. These techniques include content optimization (adding valuable content to your site), link building, and other ideas included in Google's guide for webmasters.
One site that lists some risky practices is blackhatseo.com. Its creator, awall19 from cre8siteforums, intended the site to get people asking questions at forums and I'm including it here for the same reason.
This paper from Stanford (pdf) presents a taxonomy of search engine spam (do you think there's a difference between risky techniques and spam?). (From SEORoundtable)
At the very least you should understand what the high risk techniques are so that you can report your competitors.
This post is intended to inform, and is in no way advocating the use of risky SEO techniques, especially not by people who don't know what they're doing and could ruin their businesses.
It's written with the same spirit that infused this quote from Ammon Johns, "to my philosophy, the SEO who refuses to properly inform their clients of all available techniques and the costs risks and benefits is not ethical."
I highly advise you read the Cre8SiteForum's thread, "Black hat techniques and what confuses me."