If you’ve changed your Gmail/Google account password as a security practice or to prevent unauthorized access to your account from a hacker, if you enter the old password accidentally on Gmail sign-in page, then email service denies you to sign-in and shows,when password for that account was changed on the page with the “Didn’t changed your password” link at the bottom which takes you to the old password used page.
The page explains that you attempted to sign into your account with the old password, sign in back using recent one and with an helpful link to password assistance page to regain your account access if you actually forgot the new one.
If you don’t know, Gmail or any other service doesn’t allow to use old passwords again. I haven’t come across this before, and actually don’t know about this, that’s why sharing.