I have an example in mind, I'm not sure if it qualifies for a “quantum effect” in your mind, but in my mind it surely does.

What if we could actually see the spin of light? We should not be able to see it because it's a quatum effect, yet some animals can, like the Cuttlefish for example.

So in my living room at some time along the year, the sun is hitting a plant with a certain angle and a certain bounce on the floor, then I can see that this special light casts a shadow of the plant that is actually moving backwards from me: when I move left the plant shadow moves right, and when I move right the shadow moves left. I actually filmed it, it looks very weird, and impressive if you think about it.

So I think that I understand intuitively this mechanism, which involves spins on one hand, and on the order it also demonstrates how light is always taking all the infinitely many paths in order to arrive somewhere.

And that is a so called quantum effect, right there in plain sight.

Here is my video: New video by Thomas Godart