We saw this impressive trick of perspective at Palazzo Spada in Rome. The arcaded gallery in the garden looks to be about 30m in length, and stretches off to a statue in the distance. However it is an optical illusion. The gallery is only 7m long, but looks longer because the distant columns are actually smaller, the floor slopes upwards, and the statue is quite small.
The tour de force illusion was made by the baroque architect Francesco Borromini, who also built some of our favourite churches in Rome. There are some more illusions in the square outside Palazzo Spada, including the mock window below.