The classic impossible triangle is constructed from three orthogonal aluminum bars. The front bar has a cutout which is aligned with the rear bar so that the triangle appears closed when viewed from a particular point (or along a short line), forming an impossible figure.
This gizmo bothers people more when it is set up with a pendulum apparently swinging through the "solid" metal of the triangle. The photograph shows the triangle with the pendulum at its equilibrium position along with the view of the triangle from an appropriately positioned camera.
Photographs of actual 3D realizations of a number of Escher art works will be found at the "Escher for Real" web site.