Markdown plugin misbehaving

On a site I’m currently working on the Markdown plugin for some reason refuses to render links and images correctly. Paragraphs, bold, titles etc.. are all correct but somehow all links and images get botched. The plugin turns:

[a link](http://www.apple.com)

into this html:

<a href="http://a">http://www.apple.com</a">a link</a>

Any ideas? I’m baffled as I’ve used this plugin on several sites without a hitch before…