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Unlikely, yes. Unheard of, no. One year ago I thought nothing of running servers on RAID5. Then over about 9 months I had two servers suffer dual-drive failures. On one, the second failure occurred during rebuild and I was able to recover everything since the nightly backup. On the other, the drives failed within seconds of each other and nothing was recoverable.
I was already keeping two spares on hand. I was set up to get e-mails of problems well in advance. And NEITHER of the second-to-fail drives had even hiccuped before.
Now I run RAID-1 or RAID-10 *only* though I'd consider RAID-6 if I could afford newer 3Ware controllers.