The blackest of black pigments is a substance called Vantablack. It reflects almost no visible light, so anything painted with it looks like a 2D cutout of nothingness.
British artist Anish Kapoor licensed the worldwide exclusive rights to use Vantablack in art, which makes him kind of an asshole, but we've already complained about him on Boing Boing and that's not the point of this post. The point is that Kapoor has n work of art at the Serralves Museum in Porto, Portugal called Descent Into Limbo. It's an eight-foot deep pit and because Kapoor painted the interior of the pit with Vantablack, it looks like a two-dimensional black circle painted on the floor of the museum. You can guess what happened next. A 60 year old visitor (probably the kind of guy who can't pass by a "wet paint" sign without poking his finger at whatever the sign is referring to) ignored the warning signs and stepped into the hole, falling eight feet to the bottom of the pit. He was hospitalized, treated for minor injuries and released.