From that link:

From experience, we have found the following five SMART metrics indicate impending disk drive failure:

    SMART 5: Reallocated_Sector_Count.
    SMART 187: Reported_Uncorrectable_Errors.
    SMART 188: Command_Timeout.
    SMART 197: Current_Pending_Sector_Count.
    SMART 198: Offline_Uncorrectable.

That's good to know, I might start tracking that. I manage several clusters of servers and hard drive failures just seem pretty random.

I've had several hard drives that started gradually increasing a reallocated sector count, then start getting reported uncorrectable errors, then eventually just give up the ghost. Usually whenever reallocated sectors starts climbing a drive is nearing death and should be replaced as soon as possible. You might not have had corruption yet, but its coming. Once you get URE's you've lost some data.

However, one time a drive got a burst of reallocated sectors, it stabilized, then didn't have any problems for a long time. Eventually it wouldn't power on years later.