Assume the following within an environment that has an account lockout threshold set to 3 - meaning that if 3 incorrect passwords are entered, a user account becomes locked:
User A types 2 incorrect passwords at the Windows logon screen - entering another incorrect password would cause their account to be locked at this point
From within the ADUC MMC snap-in, a domain administrator navigates to the Properties dialog for User A's account, and from the "Account" tab checks the "Unlock Account" checkbox and hits the "OK" button
Considering that entering a final incorrect password for User A would have caused the account to lock before the actions carried out as part of step 2, can User A now enter an incorrect password without their account locking - does ticking the "Unlock Account" checkbox when an account isn't locked reset the count of incorrect passwords that have been entered?
No, it does not. You would want to increase the number of failed attempts before an account lockout occurs or reset the user's password to something they can remember. I cannot think of any use case where unlocking an account that is already unlocked to reset a failed password attempt counter would be required.