Per my discussion on the forums:

Issue

When I use a single backslash at the end of a line within a Link Reference Definition's link label, it seems to invalidate the entire construct. However, when 2 spaces are used instead, it parses fine.

Expected Behavior

Per discussion with @jgm, both these Markdown snippets should provide a link that contains a hard line break in a validly constructed link.

Discussion

Using 2 space characters at the end of the line in a link:

[  
foo]: /uri

[  
foo]

provides the following HTML:

<p><a href="/uri"><br />
foo</a></p>

However, if I use the blackslash at the end of the line instead of the two space characters:

[\
foo]: /uri

[\
foo]

I get:

<p>[<br />
foo]: /uri</p>
<p>[<br />
foo]</p>