@Double disk failure
The failure of more than one item is far, far more likely than the usual calculations for redundancy assume!
That is based on "independent random failures" and not allowing for an external influence (PSU surge, over-temp, etc) stressing multiple items, or a bad batch of some component causing much higher failure rates as well. Also the strain of a RAID rebuild on traditional HDD's head servo can provoke them to croak before redundancy is restored, though that should not matter for flash.
On a personal note, we have a Sun/Oracle 'open storage' system configured with dual redundancy and tested it for our acceptance by pulling 2 HDD from a RAID set, and it failed. More than once, and in one case trashing some files (at least ZFS told us which ones!). Years and several firmware revisions later Oracle has not attempted to find the actual cause, and assures us that because they have not replicated it recently that somehow it must be fixed by other code revisions.
Can someone remind me of who said "if you don't have 3 copies of your data, you don't really have your data"?