Recently I was reading about a Windows 8 problem faced by a Windows 8 user. The problem was weird because no matter how correctly he typed the password during Windows 8 logon, he would get the password incorrect error. I was able to reciprocate the error message on my installation of Windows 8.
I setup a long password of my Microsoft account and then tried to log into Windows 8 with that long password. Although I was typing the correct password (thanks to the little eye icon which shows what we have typed in the password field) I would get the error message that the password was incorrect.
I have also setup a local Windows account on my Windows 8 computer. I set a very long password of that local account and tried to login. The login was successful. This meant that there was a maximum password length for Microsoft account holders only.
Upon further investigation, I found the following screen:
You can see that when I want to change my Microsoft account password through Windows 8, it shows the following message:
To make your password more secure user 7-16 characters and combine uppercase letters, lowercase letters, numbers, and symbols.
Although the combination of uppercase, lowercase letters, numbers and symbols are not a hard requirement but the minimum length of your Microsoft account should be 7 characters and the maximum length should be 16 characters.
If the Microsoft account password length is more than 16 characters, you will not be able to login to Windows 8. You can change your Microsoft password in order to get in line with the password length requirements of Windows 8.
You may also be interested in Windows 8 password recovery options.