In one scene, when Leonardo's facing the camera, you can peer into his mouth and see the background through a hole in it. We get a repeat of this from Mikey when he talks about the show almost being over.
Raphael's mask is slipping off of his face at the very end of "Don't Talk To Strangers", and right before "I Love To Ride The Carousel".
For some reason, a ukulele sound can be heard from the keyboard during the friendship song.
Nods and References
The Turtles once again mention April O'Neil and other characters from the show without them actually appearing.