A Phoronix reader, Zoltán Tősér, wrote into Phoronix today about an unpatched EXT4 data corruption bug present as of the Linux 4.0.2 kernel. He explained, "several people were affected by an ext4 data corruption bug in Linux 4.0.2. The bug is reported to be unpatched even in the most recent stable version, Linux 4.0.4. We are not sure what exactly triggers the problem, using a RAID setup seems to have something to do with it. It is reported to affect multiple distributions, including Arch, Debian and Fedora, so it seems to be an upstream problem."
There's this Arch Linux thread discussing the Linux 4.0 EXT4 corruption issue with multiple users being affected. There's also this potentially related patch and this related Debian bug report. Fortunately as the problem seems to be somewhat widespread at least among multi-disk EXT4 users, the issue should get addressed in a stable Linux 4.0 point release in the very near future.