For the chase scene at King Louie's Palace in The Jungle Book (1967), the animators thought: "Hey guys, we don't have to animate anything new! Let's just break out some 20-year-old animation from Mr. Toad & trace over Ratty, Moley & Mr. Winkie with King Louie, Baloo & Bagheera! & don't forget to replace the deed to Toad Hall with Mowgli, alright fellas?!"
Thanks & remember to leave comments!
*UPDATE* - Hans Perk & Michael Sporn both let me know (at almost exactly the same time!) about a translated Russian article about Disney animation reuse. You can find the article here: http://www.prodisney.ru/index.php?page=clones.php . Part I is translated in English, but if you want to read parts II & III, you have to translate the webpages using BabelFish.
Thanks again Hans & Mike!