I once did a RAID data recovery on a system with a high-end Intel raid controller. The controller failed and they sent a new one - only the new one couldn't assemble the raid properly. It turns out that there was a flaw in the logic for where parity was stored. Normally parity is spread evenly across all the drives - not on this version. I had to reverse-engineer the crazy raid pattern and write a program to deraid it. It had gone undetected - as long as it was running.