John Archibald Wheeler first proposed a thought experiment known as the delayed choice experiment.

Wheeler's "delayed choice", in Quantum Theory and Measurement, edited by J.A. Wheeler and W.H. Zurek, Princeton Univ. Press (1983)

Imagine the original double slit experiment but AFTER the "particles" go through slots, THEN is is decided to observe BEFORE anything hits the screen. What would happen?

If we observe after this, then we see that they came as particles through a slot BUT before there was observation, they would have come through as waves. So by observing after the fact, it seems to have changed the past in way that it caused them to come through as particles instead of unobserved waves.

It is a great "mystery" why this happens...why observation at a distance can remotely influence "quantum particles" to change their properties and even a greater mystery how observing after the fact seems to change what happened before. It seems that in the quantum world, the properties of time as we see it do not apply.

To rephrase this concept... if it is decided to observe which slots the photons come through AFTER the photons come through the slots, it will be observed that they have come through as particles. BUT, BEFORE there was observation, we know that with no observation, they come through as waves. So by observing AFTER they come through it changes the past so that they came through as particles even though without observation before they came through as waves. So the past was changed it seems.

Please see this site for a great walk through on the delayed choice experiment. Wheeler's Delayed Choice Experiment"