A lot of organisations set up security policies so that users will be locked out if they enter the wrong password too many times. The idea is to prevent brute force attacks, where an attacker could sit there all day running through the dictionary until they guess the correct password. The downside is that this can lead to Denial of Service attacks, i.e. someone could deliberately enter the wrong password in order to stop other people from logging in. A better solution is to have a throttle, e.g. if you enter the wrong password then you have to wait 30 seconds until your next attempt. That would slow down an attacker without being a major inconvenience to legitimate users. Unfortunately, Windows domain controllers don’t support this natively, and I haven’t come across any third party software that does the same thing.
Leaving aside deliberate attacks, sometimes this can happen by accident. A common cause is that someone changes their password, but their mobile device still has the old password (to access email), and it automatically makes enough attempts to lock the account. There are some other common causes linked to a recent password change, e.g. if there is a scheduled task or a service running under a user’s account, or if they have mapped drives or cached credentials for websites.
A while back, I came across a case that was a bit more interesting. The root cause turned out to be a mismatch in authentication protocols, so the error messages were misleading: there was never actually an incorrect password! Read on for the technical details.