There are a number of entries in Mantis for Squeak about this.
My feeling is that more than one preocess is running the world loop such that one starts doing the loop, another starts before the first is finished and ends (setting to nil) then the first encounters this.
The world loop is not thread safe.


From: Hilaire Fernandes <hilaire@ofset.org>
To: Pharo Development <pharo-project@lists.gforge.inria.fr>
Sent: Wednesday, 25 February, 2009 12:04:24 PM
Subject: [Pharo-project] MessageNotUnderstood: UndefinedObject>>includes:

I have this kind of error which appear randomly when I am
instantiating and displaying in real world a Morph.
Next from the morph is deduced Form field to use as the basis of a PNG
image for Seaside canvas.
It is easy to fix (or a least prevent the error) but it is system
land... so I am curious to read if other meet this problem.

See debuglog
--
http://blog.ofset.org/hilaire