A few know that in an implementation of RAID 1 is required to read data off the second disk as well. Otherwise one or even several bad sectors may appear on the second disk of the mirror.
In this situation your RAID 1 doesn't have redundancy any more and you do not know it, contrary to the principle of monitoring the array status. There may be a similar problem with hot spare drives.
One of the causes of this problem is the array being idle most of the time. To make sure the array is still redundant you can read sectors from both disks and compare it. If your RAID operates principally under heavy load you do not encounter such a issue.
In Windows, at least in server versions up to and including Windows 2003, software RAID 1 in most cases reads from the first hard drive, rendering Windows RAID1 arrays unreliable