I just had a user unable to change his password on a Windows 2008 domain. It was giving him a cryptic message about complexity requirements even though he was certain his chosen password was meeting them. I tested it myself and confirmed.
It seems his last password had been set too recently per a Microsoft-recommended default of something like 10 days if I recall.
He asked of me a very good question, which I couldn't answer: why would there be a minimum password age? How could this reasonably benefit security? He also pointed out that one might discover their password to be compromised within this 10 day period and not be able to change it!
Would there be any valid reason to enforce a minimum password age?