It's probably quite unusual, but if Thor sizes overlap, KJ can pick a local mirror copy as in effect the primary copy for a KJ lookup, such that a slave handles that part locally, and others send requests to it.
However, if the mirror copy is missing, the requests fail.
In the case where this was seen, it was because the index was sprayed and the meta data said replicated - but the file had never actually been backed up/mirrored.
It should validate that the copy it's mapping to exists.
It would a similar problem if the primary file was missing and the KJ mapping mapped to it, but the mirror copy was present.