Always check what your RAID controller says when a disk failure or other array failure occurs. You should verify by testing that such messages are difficult to miss.
Error messages displayed during the boot sequence are no longer useful as the uptime is now measured in weeks even for the home PCs.
If the controller doesn't report error messages or for some reason you don't take prompt action to restore the array redundancy once the disk failure has occurred, then there is no point in running a RAID. Using hot spares alleviates the problem for disk failures, but not for a silent controller malfunction.
On one of the forums someone told a story about RAID 1 failure where one of the member disks had failed and later it was found out that the second member disk contained the two-month-old data. Two months before the disk failure, the controller lost the array for some reason but didn't report the error. So nobody bothered to restore the redundancy. As a result, when the only remaining disk failed, there was no redundancy and all the data had been lost.
Remember that the RAID recovery may be difficult if the array is seriously out of sync.