Template Suggestions Causing Undefined Variable
hook_preprocess_page() is run for any page, even non-node pages. If you're getting that error it means your trying to access $node in the page.tpl.php file when you're on a non-node page. As no node is available on such a page, the variable won't have been set.

Because of the delay that can happen when messages are set in template files, you might only see this message when you've left a non-node page, and navigated to a node page. So it will look like it's that particular page complaining, when in fact it was the previous one.

You should be able to fix the problem by checking the $node variable is available in your template file as well as in the preprocess function. e.g.

&lt;?php if (isset($node)) : print $node-&gt;something; &lt;?php endif; ?&gt;