Time travel makes for dumb moments, even when they are not straight in the face:
*The Terminator: Kyle Reece was sent back in time by his own son to save his mother's life (Sarah Connor) from being assassinated by the robot sent back in time to kill her. Kyle and Sarah have sex, then you know there is a time loop started by.... unknown reason for Kyle to originally go back in time and have sex with John's mother
*Back to the Future: Marty goes 30s years back to the past, he changes the attitudes of both his parents, and that affects his time. To think about it, does Marty in the new timeline have any reason to go back to the past? What's going on? How is everything the same after Marty going back in time?
*Back to the Future Sequels:They go 30 years to the future to fix a mistake Marty jr makes, and by the end of the third movie Doc gives the speech "The future didn't happen, you make it as you go".
Aaaaahh, ok