Security Tip
Hi folks, When you design your sophisticated "Change Password" feature in your next application make sure to retain the old password so the user can't re-choose them anymore..
You can keep a history of passwords and don't allow to use them again.
[UPDATE]
The reason you may wanted to do this..if your password base compromised you wanted to change the password for your clients to something tempporary and also prevent them from changing this back to the old compromized one, however you also will benfit from this stratigy to force users to never having the same password they already changed - why they change their password on the first place - to be using it again...