Watch for duplicate content on your site...

Story:

Built a custom BH eCommerce site for a particular niche. Was having trouble ranking it for seemingly easy keywords. One day i noticed that my rankings jumped (went from page 4 to page 1/page 2.) Went to the site to add some more content and discovered it was erroring out with an http 500. Fix the error and suddenly my ranks went down again. It made me investigate. Discovered that I had forgotten to redirect www to the non www version of the domain name. Doh! That counts as duplicate content for Google. I was getting penalized. I checked the index and sure enough, both the non www and www versions were indexed. Quickly 301ed www to non www and problem solved.

Moral of the Story:

Look for stupid mistakes like that, and ensure that your content is unique. Make sure your www is properly redirected to non www or the other way around.

You may get rank loss because of www and non www versions. But you won't get penalized by that. If you faced penalty it may be for some other reasons.

Yeap true @scorpion_king

I had few client's who didn't redirect to non-www version but they always focused on www version to do link building and due to that their primary website URL was www version only which google considered to rank for.

But still, configuring your website with one version will help a lot in terms of uniqueness.

That's never caused Google to hit me or any of our clients in the past, but good tip I could imagine it happening, good to be safe.

In google WMT you can tell them what your Preferred site domain is. If tick the box incl. the www they will only index the www domain and not the domain without the www.

Sorry i didn't mean penalized per say, i meant rank loss.