The following is NOT only delisted demo. 
It is also an obvious beta of the infamous Sonic the Hedgehog (2006) (referred to as such to avoid confusion with the 1991 game).
It had major improvements compared to the original game. The reason why the build wasn't released 
as the final game was due to some bugs making it very easy to perfect stages in the game 
by tricking the score counter (see here for an example: https://www.youtube.com/watch?v=f6ynxFd89v4).
Therefore they had to release the most "stable" build and resulted in the unfinished
product that we saw on the shelves, and they removed the demo because it had nothing to
do with the final product.

You can see Sonic Retro for the long, long list of differences between the "final"
and the demo, or also this video which shows a few of them: https://www.youtube.com/watch?v=I-GNXENgM5A