There is a whole body of serious research into optical illusions. When you start thinking seriously about dataviz, you need to bear this stuff in mind. It would be easy to put something together that accidentally looks distorted because of one of these effects. Akiyoshi Kitaoka is a prominent researcher in this field (read: absolute legend!) and creator of some excellent examples. There are many on his facebook page and now they are coming to twitter too. Below is one creation called Red Illusion from 2007.
You see the grey square in the bottom left? See the nine dusky red squares? They are all the same colour. Yes way.
See how the red squares are shaded from dark to light? They’re not. Would I lie to you, baby? Well, yes. They actually are shaded, but suddenly you’re not sure you can trust your eyes any more, can you?
Not only are these fun to learn about, they really matter too for data visualisers. Kitaoka-sensei has published several books of illusions which you can find via his webpage. It’s never too early to stock up on Christmas gifts.