The famous quantum double slit experiment, which about particle wave duality. You pass it through a slit, depending on whether there's observer in interference or not, it ends up in a wave pattern andup individual photons. Einstein was a realist. At the time when the founders of quantem theory were trying to develop a good explanation of what precisely was going on with these experiments in quantum theory,. einstein rejected all of them on the basis that they weren't realistic. And he was right to do so because none of them made any sense. To this day, none of the other alternatives make any sense.