Even if your RAID is supposed to be fault tolerant, backup often.
Neglecting the backup is by far the worst and the most common mistake people make.
They think like "Hey, we got RAID, it is fault-tolerant, made by the best vendor
out there, cost us like a small car, so it must be reliable.
We do not need backup anymore".
Although the RAID is redundant with respect to hard drive failures,
there are still issues that may bring down the entire array,
requiring either a restore from a backup or a RAID recovery.
The RAID controller itself is a single point of failure.
The most simple way to get around this is to get rid of the RAID controller
by implementing software RAID, but then the software and drivers become the
single point of failure.
The failures of the hard drives may be correlated if they have a single root cause (like a power supply failure involving overvoltage).
This is called common mode failure.
Natural disasters, like floods and hurricanes, are known to take down entire facilities, no matter how redundant a particular system might be.
This is why offsite backups are used.
Last not least, RAID does not protect you against a human error.