In the double slit experiment, the bar with the two slits is a lens.
If you change the location of the lens it changes the focal point
which changes the picture you see on the screen and the results
of the experiment. You can go a step further and put on the wrong prescription
glasses when you view the results of the experiment, this will change the entire
history of the experiment and change the results you observe.
On to entanglement.
You have two entangled electrons, you measure one and determine the spin.
When do you think you know for sure the spin of the other entangled electron?
Answer: When you measure the other entangled electron.