You may notice in all Disney Princess merchandise, when pictured together, the princesses don’t acknowledge one another — they’re always looking away from one another or directly at the camera. Apparently Roy Disney, nephew to Walt Disney, was against the mass marketing of the Disney Princesses outside of their own stories. Eventually, Roy consented as long as they weren't aware of each other.
In The Princess and The Frog, there are various tributes to previous films. During the song "Down in New Orleans", the carpet from Aladdin is being shaken up on a balcony and Mama Odie comes across the lamp from Aladdin. A Mardi Gras parade float is modeled after King Triton from The Little Mermaid.
Walt Disney took a huge gamble with Cinderella. He had not had a hit film since Snow White and Cinderella cost nearly $3,000,000 to make. To save money when animating the pumpkin coach, the animators drew the coach to float on air so that they would not have to animate the turning wheels. Thankfully it was a huge success.
The dance between Belle and her Prince in the finale is actually a reused animation of the dance between Princess Aurora and Prince Phillip in Sleeping Beauty. The original Sleeping Beauty pair had been drawn over to become the new Beauty and the Beast pair, and this was done because they were running out of time during the production of the movie.