Constrained implementation of exclusive C14N -- waht breaks?

To pick up on the question that Scott asked today, what are the  
concrete examples of things that break if the constrained  
implementation of exclusive C14N is used (not just as an optimization,  
but as the *only* canonicalization available)?

Since you seemed to have some ideas on the call, are there concrete  
examples that you can give?

http://www.w3.org/TR/xml-exc-c14n/#sec-Implementation

Thanks,
--
Thomas Roessler, W3C  <tlr@w3.org>

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