From Sonic Retro
Drift in Hubs
After a low-to-ground Jump Dash, you will have sufficient speed to drift, though the shoulder buttons will be unusable. This will charge a neutered variant of the Air Dash for use.
3D in 2D areas
Due to the way 2D-to-3D shifts work, by attaining sufficient height and/or simply boosting in the right place, you can skip many transitions, playing 2D areas in 3D. Two are noted below:
Windmill Isle Act 2
By getting to the broken bridge leading to the final 2D section and climbing the spiral of Spinners, you can perform an Air Boost into the middle of the area, skipping the transition and playing through the area in 3D.
Rooftop Run Act 1
After the section where you defeat enemies to lower a pulley, walk back in the 2D section, and skirt the edge of the house's rooftop. You can make it into 3D in the 2D area, or take a large shortcut through nonsolid level geometry.
Go Through Surfaces
Perform an -- (360)/-- (PS3) combo, but cut it off with a shield as you finish the spinning part of the last attack. You will, depending on your timing, either fall through the floor or walk through the wall in front of you.
Grab After Uppercut
Out of Level
Randomly, when loading, the PS3 version will crash.