I swear this is not just my imagination. This has happened to me a few times in the past few weeks.
I attended a instructor-led course at Salesforce HQ and we were provided with a trial account that we were asked to set to our own e-mail addresses but also set the password to an instructor-defined password so he could get into the accounts as needed. I set the password to what the instructor asked for. The next day, I tried to log in and the password repeatedly did not work. I reset the password, and when I tried to enter in the password that kept getting rejected, and the only password that's ever been on the account, it said I could not reuse an old password. This happened again on the third day of courses before settling down.
Just recently I generated a sandbox in a Salesforce org and created a new user in that sandbox. I set up my password and definitely knew what it was. I logged out, logged back in and the correct password was rejected. I reset my password, and when I attempted to set it to the password I thought was on the account but which it rejected, it said I could not reuse an old password.
Are the dwarves going crazy or something? Either I got the password right or I didn't, and if I didn't get it right, there's no reason the password I thought was right should be getting rejected as a duplicate old password. Is there some other explanation?