This condition is checking that if the shardVersion attached to the command is ChunkVersion::IGNORED() for certain nss, the authoritative knowledge of the shard must state that the flag allowMigrations is unset or if this flag is set the collection must be unsharded. However this statement is wrong: it could totally happen that we process an update/delete operation that was targeting more than one shard outside a transaction (i.e. this request had a shardVersion : ChunkVersion::IGNORED()) while executing a DDL operation that had already stopped migrations for a certain sharded collection.