When we see Waxer, his helmet has cartoon image of a Twi'lek child. The child is Numa, the Twi'lek girl that Waxer and Boil found and rescued in "Innocents of Ryloth" (S1E20)
First episode where a clone, Rex in this case, mentions Count Dooku by name.
The 'Jedi fortune cookie' message that opens the show reads "Our actions define our legacy."
The 2020 anthology book "Star Wars The Clone Wars: Stories of Light and Dark" features a story based on all four parts of the Umbara arc that is told from Rex's point of view. It is written by Yoon Ha Lee and titled "The Shadow of Umbara". The audio-book version is read by Clone Wars voice actor James Arnold Taylor.