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Let me know if it works Mike. There is actually a third possibility which is;

Some page(s) might generate a dynamic URL only upon being visited by a browser/search agent. If that's the case, then you can set up an event tracking through your website in conjuction with Google Analytics and track teh refferer;

_gaq.push(['_trackEvent', 'Error', '404', 'page: ' + document.location.pathname + document.location.search + ' ref: ' + document.referrer ]);

After you collect some data (Submit your website to Google WMT or wait for next MOZ visit) you can export and run your filter.

The alternative to this method could be one of the 2 following;

1. enabling extreme debug/log mode on your programming platform and collect logs for further processing. You can run a small Python script to find the RegEx pattern. I advise to setup a demo copycat of your website on a subdomain and then run this experiment. You can then submit the demo sub domain to Google Webmaster tools and wait for the crawlers.
2. Reconfigure your webserver logging (httpd.conf if using Apache) to log more details. Make sure you turn back into to the normal data collecting configuration to avoid storage consumption.

Good luck,

Ali

Hi Mike,

Hope all is well. There are two things that might have made this confusion. Either you have some outdated links somewhere on your website that are leading to the custom 404 page or some external link is pointing back to your website with a wrong URL or missing product. In order to find the link (I say so, because a crawler has to hit a link to crawl so there is definitely one), you can use tools like Ahrefs link analysis and see what is pointing where. export to an excel and filter based on a RegEx you'd make out a 404 generating pattern you already have with Moz or Google WMT. You find one and you'll know where they are coming from and how to fix them. You'd be able to write custom redirects in your htaccess if they are not many. If they are many though, htaccess could slow down your website and the best way would be a back-end base redirect either custom coded or through a plugin based on your platform. I would start from

1. my error_logs in webserver logs and match them with WMT and Moz report.
2. download CSV and import to excel or program of your choice
3. filter based on the pattern
4. Match it with where you've found the link through Ahref
5. and Voila, now you know exactly how to clean them up

Hope this helps Mike,

Have a nice day,

Ali