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The validator.w3.org doesn't parse the phpbb.com main page correctly. shows an error "end tag for "SPAN" omitted, but its declaration does not permit this." and here is the quoted line '..=375510" class="siteTextMed">[Olympus/phpBB3] Opening the Bug Tracker</a></sp' where the / is outside the tag but the parser doesn't know that. There really are errors in the page but this is not one of them, but the validator shows it is.
I can't reproduce anymore as the test case document given has changed, but given how broken the markup on that page is at the moment (lost of non matching closing and opening tags), I can only assume that at the time of the report, the error was either 1) accurate or 2) cascading from another error earlier in the source. It is generally a good idea to fix errors sequentially, since cascading errors tend to happen.