A cameo role or cameo appearance (/ˈkæmioʊ/; often shortened to just cameo) is a brief appearance or voice part of a known person in a work of the performing arts, typically unnamed or appearing as themselves. These roles are generally small, many of them non-speaking ones, and are commonly either appearances in a work in which they hold some special significance (such as actors from an original movie appearing in its remake) or renowned people making uncredited appearances. Short appearances by celebrities, film directors, politicians, athletes, or musicians are common. A crew member of the movie or show playing a minor role can be referred to as a cameo as well, such as Alfred Hitchcock's frequently performed cameos.

By extension, the word cameo appearance can refer to any appearance of a person or characterd in a work in which no one would expect to see. Thus, Uderzo and Goscinny in some of their stories of Astérix, or even Edgar Pierre Jacobs (the comic-strip artist of Blake and Mortimer) in three adventures of Tintin by Hergé (Tintin in the Congo, cigars of the du pharao et King Ottokar's sceptre) and Hergé himself in several adventures of Tintin. 

source : wikipedia

Referring to Candy, she appears in several animes:

Fushigi Yuugi

This cameo happens at the end of the 5th volume of the third Fushigi Yuugi series OVA. Tamahome and Miaka are looking each other, and suddenly, they turn into Anthony and Candy!  ! ^________^ : a tribute to a cult series to another cult series!... ^____^

Magical girl (Episode 2)

Wtach the crowd carefully from 4:44 mn, and you'll see our little pigtailed heroine behind the artist!  ^^


The sword of Paros, the manga of Yumiko Igarashi

The Sword of Paros (パロスの剣 Parosu no Ken) is a 1986 shoujo historical fantasy manga composed of three volumes written by Kaoru Kurimoto, a science fiction author best known for Guin Saga, and illustrated by Yumiko Igarashi, best known for Candy Candy. The plot of The Sword of Paros is also strongly centred on the romance of the protagonists, one of whom, like Oscar in The Rose of Versailles, is trying to pass as a man.

In this manga, Yumiko Igarashi and the author, Kaoru Kurimoto, had the fun of inserting a short scene in which appear Candy and Annie, but also Terry, Anthony, Archie and Stair, who talk about their wish to participate in the tournament to which the best knights of the region are going to fight...