What is this: data-wplink-url-error=”true” and why is it added to my links?

WordPress sometimes adds this to links created by the visual inline link editor:

data-wplink-url-error="true"

Why? What does this do?

When you’re inserting a link in the tinymce editor, there is a piece of javascript that checks whether the link looks like a url. If it doesn’t it inserts this string into the anchor tag. Later on in the code, this tag is tested for to enable issuing a warning to the user that the link is probably invalid.