A customer has discovered that Grouping Search results by space omits some results. I haven't looked into which results exactly get omitted, but it's nothing really obvious like those in "personal spaces" or "emails".

For example. Search the Extranet for "drink beer" in all spaces for all content (No Mail). 13 results are returned. Group By Space gets only 10.

Similar results on a much larger scale occur when searching confluence.atlassian.com for "confluence" or "fancy". I haven't done a search that hasn't shown this bug yet.