Well, you'd better get back within three minutes, because if you don't, Sonic will exclaim "I'm outta here" ...
"Fuck this. I'm going over to one of those fanfics where I actually have genitalia."
... and jump out of the game to his death.
Now you're contemplating suicide.
Then you get the game's terrifying "you fucking lost" music, a warped tune accompanied by distorted laughter, which would be the most traumatizing thing here if your character hadn't just decided to end his life in front of you.
However, there is an extra level of dickery here, because that's Sega's specialty: The game doesn't put you back at your last checkpoint minus one life, like every other death. This one ends your game. As in you go back to the main menu and start from Level 1 with no Chaos Emeralds. What's that? You were on the penultimate level with 47 lives and two continues and you left the game unattended for three minutes? Well, we hope you have fun doing it all again.
Where Nintendo actively encourages you to take a break, Sega cripples you for not playing for 180 seconds. On an unrelated note, Nintendo won that console war.