R2

Sonic CD prototype 510 level select, with level 2 missing. The final level select can be seen here.

R2 is a reference to a deleted level from Sonic CD. Proving its existence is possible by examining the game files on a PC. When examining the level files the levels skip from R1 (level 1) to R3 (level 3), there is no R2 to be found. It's existence is also proven by using the level select menu. When switching between levels it skips from level 1 to level 3, again leaving out level 2. In an interview with Jim Tretheway it was stated that the level was likely cut as it didn't meet quality standards[1].

The only thing that is known for certain about R2 is its position between Palmtree Panic and Collision Chaos. Judging from the level selection it would have been 3 Acts.

Theories

There are many speculations as to what this missing level might have been; a popular theory is that it was the ruins area seen in the uncut extended ending video, although this clip could represent Tidal Tempest, as it was the only stage not to have a sequence in the final version of the video. There is also speculation that R2 could be a desert level as a large desert can be found on the world map used in the PC version. When the bad future music is played the cactus turns into a Metal Tower.]

It's also possible that either Final Fever or Bonus Stage could have been R2 if they had been in the R2 slot at one point. There is also another Bonus Stage shown in one of the cutscenes. Although if any of these were R2 then that would mean the prototype level select screen included 2 zones too many.