(Sorry, I don't know how to respond to an email when using the digest
mode, so I'm sending a response this way.)
I have gotten this message with XeLaTeX on both Windows and Linux when
TeX is trying to hyphenate a word that has a character not in the normal
set of characters for the language of the document.
For example, the document is written in Spanish and is set to use the
Spanish hyphenation rules. There is a word in the prose being processed
that has a saltillo character in it (U+A78C). My guess is that when TeX
tries to hyphenate this word, hyphenation fails and the result is this
emergency stop message. If one either adds a hyphenation exception for
this word (via adding it to the list in \hyphenation{}) or if one
encloses it within an \mbox{}, then one does not get the emergency stop.
For some reason, this does not seem to happen on MacOS X.
--Andy Black
