I discovered an unexpected behavior (BUG) with version 4.5 when auto commit is set with openSearcher=false and nrtMode=false
I have learned that nrtMode was never intended to be public (it was for testing purpose and removed in 5.0) and this fact was verified by Shawn Heisey via email, so this bug really wouldn't be fixed, but I was adviced by Shawn to file this ticket so it can be found by others if they bump into this issue (which can have hard to track consequences).
How to reproduce:
Download 4.5 solr, using the collections1 sample, turn on 15 seconds auto commit with openSearcher=false with <nrtMode>true</nrtMode> which is default, update/add to solr constantly every second for 3 minutes, hard crash and bring solr back up, everything works perfectly with no data loss. Now turn nrtMode = false, and do the same test, you will find data loss, data are not consistently written to disk. If you hard crash solr and bring back up, you will lose data. Note that in order to reproduce this, openSearcher must be false, and nrtMode must be false.