Erase one horrible tragedy from history can be anything bad. For example 9/11. You could erase it like it never happened and nobody lost anybody that day.(messing with time though would be unpredictable)
Also, global warming is controversial to some people. But I'm assuming most of you believe that it's not. Most people in the world agree with the global warming thing. So for that one, you can prevent global warmer by reversing the negative effects.
Hope you guys enjoy the WYR. New WYR [u]every[b] Wednesday[/b][/u]. Peace.