Twice upon a dream.

Apparently, Disney animators used to rifle through old films they'd worked on to find bits of animation that they could pencil over and repurpose for newer movies. I had no idea, but it makes absolute perfect sense. If you think animating feature length movies is difficult now, imagine how hard it was when every single frame of the film was hand drawn and photographed individually. Film projectors zip through 24 frames of film per second, so a 90 minute movie would require roughly 130,000 images. I'd be recycling things constantly!

Sources: Movie Munchies