os.environ['PATH'] missing

[peterbe at gmail.com]

This is a hard question to ask because I can't reproduce the problem
other than restarting several times over until it happens again.

I'm using Zope and Plone for a website on this debian linux vserver and
when I restart it it runs some /etc/init.d/zope restart scripts and
things fail in zope because somewhere deep in there the following
raises an AttributeError that 'PATH' doesn't exist:

 foo = os.environ['PATH']

How can it not be present?
What might cause this seemingly random situation?

Has anybody had similar experiences where 'PATH' is not present in
os.environ?