Google's data highlighter tool is now about a month old and we are learning more about it every day.
Did you know that if you had duplicate content issues, with several canonical URLs for the same piece of content, that the data highlighter tool may not work?
A Google Webmaster Help thread speaks just about that. Pierre Far from Google noted that the data highlighter tool does not always work with duplicate URLs.
In response to one webmaster complaining that the data highlighter tool is reporting a URL not found issue, Pierre said:
Looking at your site, I can reach your events page using 4 different URLs.
This is called duplicate content and you can help our algorithms pick which URL to index, called the canonical URL, by implementing one of the canonicalization techniques like 301 redirects or rel="canonical" tags.
A short time after you've chosen the canonical URL, our indexing systems will update and you can highlight that URL into Data Highligher.
Now, we may or may not want to imply that if the data highlighter is having issues with this, maybe the other method of deploying rich snippets via markup may have a similar issue with this case. I wouldn't jump to the conclusion that that logic can be applied here. In the above case, the webmaster is accessing a URL Google doesn't know about. But when you use markup, Google probably knows about one of the URLs and will see the markup on one of them.
That being said, clearly this is an SEO issue that has to be addressed by the webmaster.
Forum discussion at Google Webmaster Help.
Image credit to BigStockPhoto for highlighter