Nick Arcade Episode/Chemical Plant Zone
From Sonic Retro
Chemical Plant Zone
Chemical Plant demo was being played at the background while they're talking.
Even though Tails does not appear in the EHZ gameplay, he does appear in the CPZ demo.
One of the buildings (pointed by an arrow) in Nick Arcade's version is shorter.
Chemical Plant also has a different level layout.
<- upper crossway 
-> unknown??
The above picture, created by Pelord, shows the layout of CPZ act 1 right after the upper crossway levels off. (The middle block actually doesn't have the red pipe.)
<- upper crossway 
-> P-loop
The above picture to shows the exact same location in the final version and the Simon Wai prototype. The "steps" are moved to the right, and are replaced by a quarter-circle arc. The floating platform is also gone.