sudo rejects password that is correct

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http://serverfault.com – sudo (Which I have configured to ask for a password) is rejecting my password (as if I mis-typed it) I am absolutely not typing it incorrectly. I have changed the password temporarily to alphabetic characters only, and it looks fine in plaintext, in the same terminal. I have my username configured thus: myusername ALL=(ALL) ALL I am using my password, NOT the root password, which are distinct. Just to be sure, I've tried both (even though I know the root password is not what I should use) - neither work. I have added myself to the group 'wheel' additionally, and included the following (HowTos)