Forgot to escape newlines.

lib/jekyll/tags/post_url.rb

@@ -23,12 +23,12 @@ module Jekyll
         site = context.registers[:site]
 
         site.posts.each do |p|
-          if p.slug == @post.slug
+          if p.slug == @post.slug \
-            and p.date.year == @post.date.year
+            and p.date.year == @post.date.year \
-            and p.date.month == @post.date.month
+            and p.date.month == @post.date.month \
             and p.date.day == @post.date.day
 
-            p.url
+            return p.url
           end
         end