When a merge operation on a versionable node fails, the jcr:mergeFailed property should be set with the ID of the offending node(s), leaving the rest unchanged. This should work even if the node is checked in.
https://github.com/jboss-integration/modeshape/commit/801365e69660d10c92262818d735eaf76603d246
Fixed before GA - setting to requires_doc_text- accordingly