For those using kernel 4.0.x kernels -
ext4 data corruption debian bug report (https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=785672)
more at softpedia (http://news.softpedia.com/news/Linux-Kernel-Plagued-by-an-EXT4-Data-Corruption-Issue-481699.shtml)
appears to impact RAID and other server setups.
I haven't noticed any issues using the 4.0.3 kernel on my non-server systems.