In addition to incorrect passwords, several other login issues may occur.
Ensure the keyboard on the login screen is set to English (US) rather than British, French, or another layout. See the following KB article for instructions on checking or changing the layout:
https://helpdesk.myprps.com/KB/View/98478918-chromebook-how-to-change-logon-screen-keyboard-layout
This error often occurs after a password change. The only solution is to perform a powerwash. See the following KB article for instructions:
https://helpdesk.myprps.com/KB/View/94777810-powerwash-a-chromebook
This message may appear after a password change. Entering the old password is not required. Select Forget your old password?, then select Proceed anyway.