For Sonic Mania, here is an exclusive collectible wall poster.
It's made of thick poster paper, and was only available / seems to be made for the event called SXSW (South by South West) It has Sonic, Tails & Knuckles running, flying and gliding a city/night area in the game. "Club Spin" is visible as a sign in the background, along with an Eggman sign.
What other sign is here? The signature of one of the people who worked on the game. You can see it there in silver ink over part of the back of Sonic's head. The poster must have been available at a booth or something & then if you could find the people, maybe pay to get it signed. Of course, a signed one is worth a lot more, especially because the whole thing is limited edition/event only.