require_once('lib/Mail/mime.php') failing... sometimes

Hey all, Here's a strange once, at the beginning of a function I'm including/requiring a bunch of libraries. One of the lines is "require_once('lib/Mail/mime.php')". This line is failing and I can't quite tell why. Is there any sort of debug level I can turn on to see why it might be failing? To make things even stranger, this routine is being called from a Galaxia Workflow. When it's called directly from a standalone php, the exact same require line does not fail. By "fail" I mean that any lines after the require_once line do not get executed. Any ideas?! .josh