For the 25th Anniversary Celebration in 2016, there was a Sonic Humble Bundle released. A Humble Bundle happens when you get a big deal on games with a 'pay what you want' basic principal, which then donates to charity. The games can generally be gotten on game service Steam. However, sometimes such a bundle includes more than games...such as prizes like this fun tee!
This shirt was available if you donated enough money to reach its prize tier. It has a geometric-modern-semi abstract look with the blue ball representing spinning Sonic bouncing up off a spring in Green Hill Zone. The words to the right say 'soft polycotton 50/50 blend'. Because Humble Bundles are time-limited AND this was a prize, it is destined to be exclusive & uncommon.

Limited edition tees!
These 2 shirts are limited editions to celebrate the upcoming release of Sonic Mania. Amazon partners with Sega to release these 2 tees for only the month of September 2016. (So, they'll likely end up rare/collectible) The design is pretty nice here too, even if you have lots of other tees. Studiopolis, the new zone is highlighted movie/travel poster style on the gray tee. It says "See the sights! Experience the sounds! Visit Amazing Studiopolis". If you look carefully at the bottom there are colored sihlouettes of Sonic, Tails & Knuckles in front of the city background. The black tee is called "Triple Threat Tee". It has simplistic profiles of Sonic, Tails & Knuckles, each with a thick outline.The side facing art is neo-classic, and likely new to this shirt. The Sonic Gear Store will have buttons for both BUT only while they're for sale!