Back to the future is a classic example of time travel consequences. For example biff is very abusive to Marty's dad in the begging. Then Marty goes back in time "accidentally" and gives his dad some excellent advice. He tells him to stand up for himself and the woman he likes, which is Marty's mom. He takes the advice and confronts biff while he is giving some unwanted attention to Lorain. He knocks biff out and wins Lorain and some confidence which he carries to the future with him.
      Another good example is when Doc Brown acquires some plutonium for the machine and makes the wrong people mad. Then right when Marty and doc are about to test the time machine they roll up in a vw and gets shot by terrorists. Then Marty goes back in time in order to try and worn him about it. At first the doc doesn't want to take the chance of knowing and altering the future. Marty slips a note in the docs pocket right before he leaves the past. unfortunately he finds Marty's note and tears it up before he leaves for the future. To Marty's surprise when he goes back to the future he finds out that the doc read it anyway and put on a bullet proof vest. Sometimes the consequences of time travel work out for the better.