Ever run into a situation when you thought you had sudo rights on a machine and tried to issue the sudo command and upon finding that you don’t have them..get your name added to the sudoers list by begging the sysadmin.. and then frusrated when sudo keeps throwing an error that you are not part of the sudoers list? Hmm.. that is a long sentance 
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To expire any cached security permissions, so that sudo is forced to check the sudoers files, issue the following command
sudo -k