He describes a “fun demo”—dubbed the “one for three illusion”—in which a user
is instructed to pick up and stack cubes in a virtual environment. Users are
first shown three real cubes on a table. As they don a virtual reality
headset, two of the real cubes are taken away—though three cubes still exist
in virtual space. As users reach for one of the now-illusory cubes, a subtle
visual warping causes them to correct their hand trajectory in the real
world, so that they reach toward the one remaining real cube. “You don’t ever
realize that you’re only ever touching one object,” says Suma. Users, Suma
and his colleagues reported, found the task “repetitive”—until they took off
the headsets and saw what was really happening.

ref

via gwern