Crazy rule

Following Bach & Harms (1972), a grammatical (especially phonological) rule that is attested in a language and is learnable, but has no apparent synchronic motivation, is sometimes called a crazy rule. Such rules sometimes arise through sound changes whose original motivation has been obscured by subsequent unrelated changes.

Example
In Japanese, the coronal plosives t and d are affricated not only in front of i (which is phonetically natural), but also in front of u.

Synonym

 * unnatural rule

Reference
Bach, Emmon & Harms, Robert T. 1972. "How do languages get crazy rules?" In: Stockwell, Robert & Macaulay, Ronald (eds.) Linguistic Change and Generative Theory: essays from the UCLA conference on historical linguistics in the perspective of transformational theory. Bloomington, IA: Indiana University Press, 1-21.