I've came across the same issue on linux mint debian edition with the mate desktop, although there is an additional twist to my story. Once i lock screen not only is the password incorrect when i try to unlock, but its incorrect when i try to ssh in from my laptop or after a restart and try to log in with mate desktop manager. It's like the simple act of clicking 'lock screen' in the mate menu corrupts or changes the user account password in some way completely.
I had to chroot in from live usb as root to set my account password again. So at least you can recover from it, but click lock screen again and boom! same issue.
However...i just so happened to of changed the hdd in my 2nd machine and hence have done a clean installation on it today... the issue is also present in a fresh install of lmde 201303 mate edition (both machines are lmde 201303 mate x86 edition).
Suppose though, i cant fault 'lock screen' on being able to lock the computer