Most of us have had the client from hell but we often don't like to talk about it. There is a Google Webmasters Help thread which takes us through one client from hell.
If you read through all 60 or so posts, you will see how hard this web designer is working to make this client happy. The designer tells us that the client was very happy with the design, and everything. But when the client didn't see their site in Google, soon after it was launched, the client was extremely displeased. The designer, who was not an SEO, came to Google Webmasters Help to help get the site listed.
The designer learned from professional SEOs and Googlers that the site was done fairly search engine friendly and that over time, the site will be indexed and ranked in Google. But it wasn't soon enough. The designer added more and more SEO changes, such as an XML sitemap, removed some flash, added some links - but it wasn't fast enough. The client then found the Google thread and threatened the designer that they would sue him. So the designer gave the client his deposit back and took down the web site.
Guess what? A day later, the web site showed up in the Google index, only to return an empty site. And now, I am sure it doesn't show up anymore.
We probably all had clients like this. If not, read the thread and you can learn lessons from this.
Forum discussion at Google Webmasters Help.