For proper operation of a dynamic site, all non-canonical URL paths should either return a 301 redirect to the canonical URL, or should return a 404 response. I'm finding multiple such crawling issues within ZenCart, and these issues interfere both with proper analytics tracking as well as throwing up multiple duplicate content errors within various WebmasterTools reports.

Using the rel=canonical tag is a band-aid solution, one that still fails to properly fix all of the issues. This is a common problem within many sites using faceted navigation. One simple solution is for the URL of a product page to NOT include any category information: that information is contained in the database and can still be used to build the page content, without being in the URL.