The ShareableNodeTest.testRemoveMixin() assumes that removing the "mix:shareable" mixin will always throw an UnsupportedOperationException. This is not only checking for the incorrect exception, section 14.15 "RemoveMixin" specifically states that it is possible for an implementation to support removing the 'mix:shareable' mixin:
"If an attempt is made to remove the mix:shareable mixin node type from a node in a
shared set the implementation may either throw a ConstraintViolationException or allow
the removal and change the subgraph in some implementation-specific manner.
One possibility is to replace the node with a copy that has no children (if this does not
violate the node type restrictions of that node). Another possibility is to give the node
a copy of all of its descendants (unless the resulting copy operation would be unfeasible,
as would be the case if a share cycle were involved)."
Thus, even though it is possible for an implementation not to allow removing the "mix:shareable" mixin, the test shouldn't expect that an implementation will throw an exception. For example, one particularly easy thing for an implementation to support is removing 'mix:shareable' if and only if there are no other shared nodes (e.g., the "getSharedSet().getSize() == 1").
This test could be removed, or perhaps it might be possible to test that removing the mixin (and saving) will either succeed or will throws only a ConstraintViolationException.
I marked as critical because the TCK test prevents other implementations from correctly proving compatibility.