This issue was originally reported by Dave Peterson.

Pattern derivations remove lexical representations directly from the
lexical space.  If a lexical representation is not unique then it is
possible with a pattern to remove the canonical representation without
removing all representations, thereby leaving the value in the value
space but without a canonical representation.  This is related to
RQ-129 (#eliminate-canonical).

See (member-only link)
http://lists.w3.org/Archives/Member/w3c-xml-schema-ig/2002Nov/0186.html

This item was discussed in the meeting of 2003-11-21
(http://lists.w3.org/Archives/Member/w3c-xml-schema-ig/2003Nov/0050.html).
Paul Biron to produce phase-1 proposal.

This item was discussed, and phase-1 agreement was reached, in the
meeting of 2004-03-19
(http://lists.w3.org/Archives/Member/w3c-xml-schema-ig/2004Mar/0072.html).

Proposal:

Dave Peterson and subsequent thread (member-only links)
(http://lists.w3.org/Archives/Member/w3c-xml-schema-ig/2002Nov/0186.html)

Since it's not feasible to prevent the use of patterns in such a way as
to eliminate canonical representations, the only practicable resolution to
this issue is to add a warning to the spec noting that this can happen 
and that it may in some circumstances be undesirable.  Such notes have been
added to section 2.1 (Datatype) and just after the definition of the 
pattern facet.  With those notes, this requirement has been discharged.