Scenario: master/slave replication, master delta index runs every 10 minutes, slave poll interval is 10 sec.
There was an issue
SOLR-4413 - slave reads index from wrong directory, so slave is full copy index from master every time, which is fixed after applying this patch from 4413 (see script below).
Now on replication the slave downloads only updated files, but slave is create a new segement file and also a new version of index (generation is identical with master). On next polling the slave is download the full index again, because the new version slave is force a full copy.
Problem is the new version of index on the slave after first replication.