Part of Sonic's charm is finding all the secrets in a level. But if you really want to find all the secrets, you have to go a step further.
That's what the Boundary Break series on YouTube does, leveraging noclip cheats, glitches and debug menus to take control of the in-game camera. And since Sonic Mania comes with a debug menu, which you can access (on the Switch) by pressing X once you're in a stage selected from the level screen.
There isn't as much junk or easter eggs off-screen in Sonic Mania as, say, the levels in Portal. But there's some neat bugs to be found, like the fact that only the surface of the lava actually damages Sonic, and there's some unused platforming segments in the boss battles.
What's your favourite easter egg or doodad typically hidden off-screen in a game?