This is the same issue as bug 2210, but opened against schema 1.1.

Consider 

<complexType name="derived" mixed="false">
 <complexContent mixed="false">
  <extension base="base"/>
 </complexContent>
</complexType>

where "base" is a complex type whose {content type} is a simple type 
definition. 

Then according to structure 3.4.2, "derived" has the same content type as base, 
which is a simple type definition. But it specified <complexContent> 
explicitly, and mixed = false. This seems confusing. 

And if mixed was true on <complexType> and <complexContent> then the rule 
actually says that "derived" has a content of a sequence of an empty sequence, 
following the particle from "base". But "base" doesn't have a particle. (Rules 
from 3.4.6 says this is invalid, but those are at the component level. Here we 
are talking about the mapping.) 

IMO, <complexContent> shouldn't be allowed to extend a complex type with simple 
content. That is, change bullet 1 of the constraint "Complex Type Definition 
Representation OK" [1] to 

1 If the <complexContent> alternative is chosen, then all of the following must 
be true: 
1.1 The type definition resolved to by the actual value of the base [attribute] 
must be a complex type definition; 
1.2 If the <extension> alternative is also chosen, then the type definition 
resolved to by the actual value of the base [attribute] must not be a complex 
type definition whose {content type} is a simple type definition; 

[1] http://www.w3.org/XML/Group/2002/09/xmlschema-1/structures-with-
errata.html#src-ct 

See:
http://lists.w3.org/Archives/Public/www-xml-schema-comments/2003AprJun/0048.html

HT proposed a resolution to R-218 which was discussed at the 2003-10-10 
telecon. Proposed errata text approved (but the WG allows him to edit the text 
to express it more formally). (This decision was in the context of 1.0.)

Link to the approved proposal:
http://lists.w3.org/Archives/Member/w3c-xml-schema-ig/2003Sep/0070.html