Timeouts. Sometimes they just happen.
Up until very recently, a timeout when trying to perform a HTML validation test for a URL would leave the test stuck as being in progress. This prevented the full site-wide test from ever showing as 100% complete, often getting stuck at 90-something percent.
Timeouts join a collection of three new test failure situations supported:
Retrieval timeouts: a timeout when retrieving the HTML of a page when trying to test it
DNS lookup timeouts: a timeout when trying to resolve the domain of a URL
Malformed URLs: sometimes a URL can't work as it is just not quite made correctly
In any of the above three cases a test will be marked as failed and when viewing the results of a test you'll get a brief description of why the test could not happen.