Section 3.7.5.2 (Mixins and Inheritance) of the JSR-283 specification (aka, "JCR 2.0") states:

A mixin node type may have one or more supertypes, which must also be mixin types.

However, our mixins have the non-mixin nt:base as a supertype.

Jackrabbit doesn't mind, but ModeShape performs validation of the inheritance when registering the nodetype.