At about 35 minutes or so into the movie, when the dogs across London are sounding the "Midnight Bark", several characters from Lady and the Tramp are visible. The first are 2 of the dogs in the pet store, who were in the pound with Lady. About 5 seconds later, right at the very end of that scene, the Tramp is visible in silhouette on top of the truck to the right and Lady appears on the street below in the shadows. It's quick - use your pause button right at the very end of the Midnight Bark scene to see them.

Lady (in the foreground) and the Tramp (on the truck in the background)