Atom feeds use <link href="$URL"/> for entry URLs, as opposed to <link>$URL</link> in RSS. The RSS extension, however, assumed that Atom feeds have the URL in <id> tag, which is wrong according to RFC4287, and in practice some feeds may duplicate URL in <id> since a URI can be a valid IRI, but they don't have to and some will argue they shouldn't (a problem with URI as unique entry id would be that the id is supposed to stay the same even if the post URL changes).
For a real example of a valid feed the RSS extension will fail to render corectly: http://blog.posthaven.com/posts.atom