In Sonic CD, if you wait for 3 minutes without touching the controller or the iPad, that will cause Sonic to say "I'm outta here!" jumping off the screen and given the person a game over.
There's hidden messages in the game. One of them is Sonic saying in Japanese style type "You are cool." A Tails with a car saying "See you next game!" A bizarre look to Batman. Sonic and Metal Sonic and Dr. Robotnik rapping and the word is saying "World's fastest DJ." And, finally, a screen of full looking disturbing Sonic with creepy music with Japanese words. The end meanse "devil" or "demon."
Here's a video of "I'm Outta Here!"
Here is a picture of the hidden messages from Sonic CD: