The best selling Sega Game in the company's history.
This game marked the first appearance of Tails. Just like Sonic, Tails's real name, Miles Prower, is a reference to speed (miles per hour).
Casino Night Zone was based on the experience of the Japanese staff working in the U.S. who would often travel to Las Vegas.
Although it is unknown what caused the mutation that gave Tails his namesake twin appendages, the concept of a fox with two tails is common in Japanese folklore. According to the mythology, foxes are magical beings with the ability to shapeshift; the older a fox, or kitsune, gets, the more powerful it becomes and the more tails it has. Kitsunes gain a new tail every hundred years until they've reached their maximum strength of nine tails. They're notoriously mischievous, so if you're walking in the woods and happen upon a nine-tailed fox, it's probably not something you want to investigate.
Song #10 on the sound test screen was from the Hidden Palace Zone, which didn't make it into the final version of the game. The omitted Hidden Palace Zone was featured in a Japanese preview for the game.