Here is an optical illusion that came about entirely by chance.
Have a look at the picture, and in particular the two frisbees that you can see.
Initially you might think that the picture has captured a moment when both frisbees are in the air. But that is not the case. Only one frisbee is actually airbourne. Can you see what is happening? A clue - look at where the shadow falls from the man's outstretched arm.
The black frisbee is in the air, and the yellow one is on the grass. But the photographer has captured a moment when the shadow of the black frisbee is in just the right place to convince us that it is the shadow of the yellow frisbee, and that the yellow frisbee is actually in the air.