If you collect all the rings during the ending sequence, Sonic will show his fist pump pose from the Sonic The Hedgehog 2 and Sonic The Hedgehog 3 ending sequence, or turn into Super Sonic if you also have all the Chaos Emeralds.
ACHIEVEMENT - HOW TO UNLOCK All Stages Cleared! - Defeat the final boss and view the ending. Contender - Upload your recorded scores and clear times for all stages. Crush Dr. Eggman - Defeat a boss for the first time. Enemy Hunter - Defeat 1,000 enemies. Golden Flash - Clear all Acts as Super Sonic. Immortal - Build up 99 or more extra lives. Ring Collector - Collect all the Rings in 'Special Stage 1' and Clear the stage. Speed's My Game - Clear SPLASH HILL ZONE ACT 1 in less than a minute. Super Sonic Genesis - Acquire all seven Chaos Emeralds. The First Chaos Emerald - Acquire a Chaos Emerald. The Story Begins - Clear SPLASH HILL ZONE, ACT 1. Untouchable - Clear the E.G.G. STATION ZONE without taking any damage.