Imagine, in the far future, you were able to to create a simulation that was so advanced that it could accurately recreate everything that has happened in the universe so far, so that it was an exact replica of history. Also, that there is a way which you can determine whether you are in a simulation or not. Assuming you are in the real universe, if you make this measurement, it will show false, however when your simulation reaches that point in history, the result would show up true, and reality and simulation would diverge. The only way to stop this would be to make a more advanced simulation that could not be detected with any method, or to have some part of your simulation check for when this test is made, and spoof the result...