This morning I woke up to find on of my networks without a database. Why? I presume it was caused by an electrical failure in the town where the network is located. If not, it might have been the upgrade to mdadm 2.5.3.
It is somewhat ironic, because I just recently finished building a small high-availability web server cluster there, and am planning to build one for databases too.
It annoys me when this happens obviously, but I had bought a quality uninterruptible power supply UPS just for this reason.