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Yeah, unfortunately, on a CMS or template-driven site, it's really easy to put a canonical tag in place that impacts the wrong pages. Hopefully, you caught it in time. The 270 notices in are system are just telling you that 270 URLs we crawled had a canonical tag pointing to a different page. In this particular case, it was a problem, but it isn't always an issue. Unfortunately, with the bad canonical tag in place, it's tough to tell why they were there before. This is usually just a notice, and non-critical, but once the bad canonicals clear out, let us know if you're still getting the notice (it may take a couple of weeks to go away).

Tags carry no SEO value.  All they do is notify your competition the exact keyword phrases that you are targeting.  Matt Cutts has suggested many times that categories alone are a best practice. Have a good one, Jared