Our current migration from MOSS to SP2013 has been interesting, needless to say.  Because I am an administrator I never see these little issues.  This is one of the latest fires I had to put out.  This popped up for different users whenever they would click on new document.   At first, I thought it was s permission issue, but that was not it.

Turns out that this is an issue with Internet Explorer 8 (of course).  The new document +  calls a javascript function that ie8 cannot recognize.  Instead of opening the web based InfoPath form, it tries to add a document.  After testing on other systems using ie9 +, Firefox, and Chrome this is the only logical explanation.

To address this issue, I had to add a Page Viewer web part to a page and link to the opened view of the form.  Not the most elegant approach, but it works.

Stay Salty!