This is something, one hopefully will never encounter. A corrupted RAID 5 stripe.
RAID 5 works by storing data in logical stripes (64 or 128KB) with parity across the set of harddisks. This is usually 3 to 6 disks. If any of the disks were to fail, it can be replaced and data recreated from the parity bits.
It is also possible to have a corrupted logical stripe or punctured stripe as Dell calls it. This when the RAID controller does a hiccup. The only solution is to backup, wipe, re-create and restore. Yes. Ouch.
Edit: 1 Dec 15
Dell has released a utility to fix this.