Going back/forth in time is the biggest paradox known to humans.
An example of this is going back in time to fix a mistake made by yourself or human kind, or whatever.
If the mistake wasn't made, you wouldn't be going back to fix it, thus it would be made.
A superstition of sorts is that the universe would collapse upon itself if such a paradox is created.
Thus why some believe time traveling should not even be attempted and is not supported by many people.