(Big Spoilers) Dr. Robotnik's Ring Racers - Invincible Cup [Master Difficulty, S Rank] 

Heartbreaking - You didn't angle yourself correctly and that costed you 1st Place.
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Master difficulty is an unlockable difficulty after collecting all 14 emeralds.
On this difficulty, The speed is set at Gear 3 just like Hard Mode but ALL Bots will be MAX Level at the start and they will NOT get easier even upon continuing. On top of that, in Bonus Round, you have NO bumpers and the timer will not refresh above 10 seconds! It's a challenge on how well you can deal with strong bots and your overall mastery of the game.
Also, on this difficulty only, Finishing 1st overall on a GP GUARANTEES a Sealed Star, rather than being within A Rank status after the last race of a GP. Perfect, considering there is 1 unlockable for getting all the emeralds AGAIN... but on Master. Sealed Stars will also be on Gear 3 speed rather than Gear 2!