The link definition names are stored in an Array.  Arrays have `reduce` as a method, which seems to be the source of the problem.  You can reproduce it with `concat` or `filter` or any other Array method, too.  I'm no Javascript expert, and I haven't dug far enough into the problem to figure out just why it happens -- especially because it begins working again if you include the optional title for the link -- but you can see for yourself that it affects the [base WMD implementation][1], too, not just SO's version.


  [1]: http://wmd-editor.com/examples/splitscreen