Effect = Cause
So you have just killed the Big Bad? Ooops, that's what created them in the first place. Trying to save the universe by ripping a hole in it? Turns out the hole was what caused the problem in the first place! Of course, what actually initiated the chain of events in question is now a complete mystery. As well as events, this can often result in objects that are never created but are instead passed around through time without any adequate - explanation - something that is often Lampshaded by the characters themselves. Kids, don't play with paradoxes.