I need to escape a closing parenthesis of the URL to fix the problem.
[Język programowania C](http://pl.wikipedia.org/wiki/C_(j%C4%99zyk_programowania\))
+
+> This is a bug in markdown version 1. It is fixed in [[!cpan Text::Markdown]],
+> which ikiwiki will use if it's installed. [[done]] --[[Joey]]