Are you asking if you can lose 2 drives back to back? Sure, anything can happen. Raid 5 allows for great availablity and performance increase for data access, but raid 5 does not back up anything. It just simply helps prevent use of your data due to a single drive hardware loss. It is not a copy of your data. You can't recover an old copy, an old revision, or simply a copy of your current work. Also, does not protect against data corruption. There are more things that could go wrong than just simply losing a drive. Virus could corrupt all your data, little sister likes watching the trash can on your desktop become full and empty as she throws files in it, stupid friend drops a soda on your machine, etc.
Also, remember, you can lose hard drive raid controller. And you can't just move the array to another random controller. You normaly have to use the exact same one and still, something could go wrong. Some raid controllers store information on board and other send configuration info to the array attached. It is a gamble when this situation arises.
Same question over at SF: http://serverfault.com/questions/2888/why-is-raid-not-a-backuphttps://serverfault.com/questions/2888/why-is-raid-not-a-backup
Need more reasons?
EDIT: Your idea is correct and could happen to anyone. I personaly have not seen more than one drive fail, but I have seen some die really close together. None of them were in that window of rebuilding, but it is technicaly a risk. But, you have a backup in case something does happen right? haha. Some people learn the hard way on this one sometimes. Raid 6 takes it to the next level with dual parity and can lose up to 2 drives. With any raid setup, the propability of failure rises with the size (# of drives) and complexity of the array. More drives = more points of possible failure
