After using your time machine to send messages in the past to help your friends and eventually developing a version that allows a person to send their mind to their past self. This interference with the time line not only causes a dystopian future, but also means that your closest and oldest friend is destined to always die at a specific time, despite the countless attempts you make at saving her life. The only way to save her, is by systematically undoing all the alterations you made, but that then means that most of human life is wiped out by the Third World War, and the woman you love is murdered instead. (Steins:Gate)
Luckily your future self informs you that you can only change the timeline for the better by manipulating the first set of events without actually changing anyone's perception of them, preventing WWIII (for now) and saving your friends (hopefully). Turns out all you really need to do is get stabbed.
Go back in time to ancient feudal Japan, meet your idol, only to find out that he's a lecherous drunken douchebag of a white guy who is, incidentally, immortal. (Heroes, Season 2)
Finally make out with the gal who's been giving you the eye for the last two seasons of the show, only for it to have never happened. (SG1, Season 10: Eternal)
Surive a plane engine crashing into your bedroom. Spend your days haunted by your future self in a freakish bunny suit. Then, when reality starts to unravel because of your innocent altering of the timeline, realize that you need to be in your room when the airplane crashes into it to save the girl you love. (Donnie Darko)
Deliberately dive into a black hole, spawning an alternate timeline where you get captured by a crazed refugee from a dead world who then goes on to kill your homeworld while you watch. Meet up with the younger version of your best friend, who completely dislikes your own younger self. Tell him that your two selves meeting up will make the universe explode, just for the lulz. (Star Trek (2009))
Get your Klingon life saved by some stewart of your family house you didn't know. See how this man tries to help you raise your son Alexander as a warrior, which Alexander keeps resisting, to catch him in the end trying to shoot your son dead. When you struggle and disarm him, it turns out he's your son grown up to your own age, trying to kill himself as a child because he caused the death of you. It was a very good episode. (TNG).
You use a temporal weapon-ship to erase your enemies from the timeline (resulting in them never having existed), with the unfortunate side-effect of said enemy needing to have existed to introduce an antibody vital to the physiology of your own race.
(ST Voyager: Year of Hell).