Question on Signature types

[Thamsanqa Moyo <calabash@rucus.net>]

Hi

The Web Services Security: SOAP Message Security 1.1 document says in 
section 8:
    Because of the mutability of some SOAP headers, producers SHOULD NOT 
use the Enveloped Signature Transform defined in XML 
Signature...Similarly, producers SHOULD NOT use the Enveloping Signature

I am interested to know if anyone can give me a scenario/example  
showing why it is a bad idea to the use enveloping signature (as defined 
in XML Signature) because of SOAP header mutability ? I have given it 
some thought and looked in literature but cant come up with any good 
example.

Thanks
Tham