Error "Mandatory property lastModifiedBy does not exist" when restoring previous version

Sometimes we get an exception when restoring a previous version of a document:

javax.jcr.nodetype.ConstraintViolationException: /content/documents/foo/foo[3]: mandatory property

{http://www.onehippo.org/jcr/hippostdpubwf/nt/1.0}

lastModifiedBy does not exist

Unfortunately, I cannot reproduce this at the moment. Under which circumstances could this bug occur?