After defeating the boss of Studiopolis Act 2, an error message pops up on the TV monitor in the background accompanied by a buzzing noise.
This was the same buzzing noise that occurred during the livestream of the Sonic 25th Anniversary Party event that Sonic Mania was first announced at, which became infamous for it's numerous technical issues.
Also the numbers 072216 displayed on the error message reference the date of the event, July 22nd 2016.