According to a post (http://news.softpedia.com/news/Linux-Kernel-Plagued-by-an-EXT4-Data-Corruption-Issue-481699.shtml) at Softpedia, there's an ext4 data corruption issue involving Linux kernels 4.0.x.
see also the bugreport https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=785672
It's apparently fixed in 4.0.4 according to the reddit thread on the subject in /r/linux. Luckily towo likes to give us the latest and greatest.
-edit- ok the softpedia article says its not, and it seems to be those running a raid setup.