In the CSDL spec (2012-03-19), section 12.1.2 "The IsDefaultEntityContainer Attribute" states:

Each metadata document that is used to describe an OData service MUST mark exactly one entity container with this attribute to denote that it is the default.

But the second CSDL example in section 17.2 has:

<EntityContainer Name="Model1Container" >
<EntitySet Name="CustomerSet" EntityType="Model1.Customer" />
<EntitySet Name="OrderSet" EntityType="Model1.Order" />
</EntityContainer>

Note that IsDefaultEntityContainer="true" is missing, which is not in accordance with section 12.1.2.