Parentheses may also be nested (with one set (such as this) inside another set). This is not commonly used in formal writing [though sometimes other brackets (especially parentheses) will be used for one or more inner set of parentheses (in other words, secondary {or even tertiary} phrases can be found within the main sentence)].^{[citation needed]}

via Bracket – Wikipedia, the free encyclopedia.

As pointed out by the XKCD blag.