Scenario: you have traveled back in time to make a minor alteration to the entire time stream. Instead of killing baby Hitler or vetoing the Manhattan Project, you figure the best way to use this mind bending technology is to switch all of Rammstein's Quentin Tarantino DVDs with VHS copies of Garfield and Friends.
Congratulations, you'd just created a time paradox, as evidenced by the existence of Knorkator: