The café wall illusion is a geometrical-optical illusion in which the parallel straight dividing lines between staggered rows with alternating black and white "bricks" appear to be sloped.
It was first described under the name Kindergarten illusion in 1898, and re-discovered in 1973 by Richard Gregory. According to Gregory, this effect was observed by a member of his laboratory, Steve Simpson, in the tiles of the wall of a café at the bottom of St Michael's Hill, Bristol. It is a variant of the shifted-chessboard illusion originated by Hugo Münsterberg.
The precise cause of the illusion is not well understood, although it appears to involve interactions between the neurons in the visual cortex which code for orientation. It is unclear whether some inhibitory mechanism is at play, or if there is a kind of computational filter acting on input from cells operating at different spatial frequencies, i.e. taking their inputs from larger or smaller areas of the visual field (Takeuchi 2005).
Philosophers have also been interested in what illusions like this illusion can tell us about the nature of experience. For example, in the case of experiencing the Café Wall Illusion, it would seem to be that the one can know that the lines are parallel whilst at the same time one experiences them as unparallel. If so, then this might count against the claim the perceptual states are belief-like, because if perceptual states were belief like then, when experiencing the Café Wall Illusion one would simultaneously believe that the lines were, and were not, parallel. This would seem to entail that one was being irrational, because one would simultaneously be holding contradictory beliefs. But it seems highly implausible that one is being irrational when under going this illusion. For discussion of this general point about whether perceptions are like beliefs, see Crane & French (2016).