The process of SEO frequently requires that you conduct periodic sweeps of the various platforms that make a website operational and either contribute to or impede search engine rankings.
Sweeps like this often include everything from assessing spider traps (where php can loop) and promote over-indexation, load time for server platforms, debugging blocked site segments, battling duplicate content issues or patching up inconsistencies in dynamic code due to performance based anomalies.
After sifting through Google Webmaster Tools a few days back (during an investigative sweep) we noticed there was a strange occurrence of duplicate content (the internal kind), creeping into our CMS (content management system) and wreaking havoc.
Fortunately, after delving in with our development team and virtually decompiling WordPress hooks and the core code looking for the source (obvious overkill to the solution); we found an interesting plugin to fix the (case of the wildcard category) problem called permalink validator by Rolf Kristensen.
The symptoms were that we were able to change the name of the category (such as SEO to SEO123) and the code would not generate a 404 error. Generally, this would not have created concern, however if you happen to have multiple plugins installed, often enough, they do not always play nice with each other.
Ideally, if you do find a critical conflict you can 1) backup your database 2) start disabling plugins one by one to determine and see if you are still experiencing the problem.
After going through this preliminary step, we still were able to change the categories (using wildcard/anything goes numbers, letters, etc.) and had to escalate the issue, looking deeper into the core code.
Another consideration is, if you are still using plugins that have not been updated for a period of time, core code updates to the WordPress can change, which can create bugs or multiple conflicts if left unchecked.
In this instance, the rewrite functionality was not toggling a 404 error, which left our site architecture open for interpretation and even worse, duplicate content. It is not uncommon that content management systems will duplicate posts or pages across multiple categories. Having the proper http status codes returned from a server header is critical for effective search engine optimization.
If 404 errors are not toggled, then it could facilitate spoofed urls or issues with categories, subfolders or pages through generating similar shingles (groups of words) across multiple pages. Search engines in turn have the tendency to deindex duplicate pages and discard them into a secondary / supplemental index.
If pages are supplemental, they are removed from the main search index and any significant rankings those pages garnered also disappear with those pages. For a 30 page website, this is of little concern, however, for a website with hundreds or thousands of pages, tags, feeds and aggregators, this type of disruption can be a costly setback.
Also, considering that WordPress often places the words /category/ then the category name by default, you should remedy that by using a plugin such as top level categories which is ideal if you are using the /%category%/%postname%/ custom option in your permalinks settings to theme and silo your content with a logical site structure.
After updating the plugins (and cleaning house on disabled plugins) something you should do, if they are not in use, the permalink validator wordpress plugin worked just fine in preventing the wildcard categories.
A great place to sniff out errors like these is in the html suggestions tab in your Google webmaster tools region. If you see duplicate meta tags and meta titles then you know the typical rewrite functions are malfunctioning.
You also want to make sure that all canonical issues are resolved within your website to resolve to a preferred URL structure either http:// or http://www. And then your domain name. A quick method to determine if your website is suffering from duplicate content in the index is to use a site:domain.com then use site:www.domain.com site:domain.com/ and if all three are returning the same number of indexed pages, then canonical settings are ideal.
Another must-have SEO plugin for WordPress is SEO Ultimate (created by John Lamansky & SEO Design Solutions) which currently performs over a dozen unique features in one stable plugin allowing you to do everything from rewriting titles, tags, categories, tracking 404 errors, reducing pagination issues and duplicate content with canonical tags and much more.
The moral of the story here is, the more moving parts you have, the more things that can possibly break under the wrong circumstances. Make sure you take the time to update your plugins and keep abreast of critical security patches for WordPress as they arise. As the saying goes, an ounce of prevention is worth a pound of the cure.