http://www.macdevcenter.com/pub/a/mac/2004/02/24/bash.html?page=2
It's a bit down the page, so here's the quote:
The shell and related tools require certain variables to be defined (such as ), but other tools may need their own environment variables. In the event that you are missing a required variable, chances are the related program will fail safely and let you know what needs to be defined, as does Apache's fop:
http://www.macdevcenter.com/mac/2004/02/24/graphics/fop.jpg
We must be doing something right, if we're being used as an example (even if it is referring to how nicely FOP reacts when it fails)!
:-)