iOS 9.3.5 is a small update for iOS 9 users with a short list of patches for some serious exploits. And while the iOS 9.3.5 update is working fine for some users, others are running into trouble with Apple’s last iOS 9 update.
I once updated whilst on charge and just assumed that the battery would still be charging during update - it wasn't! I only lost a week of stuff - so now I backup before I update.