There is a minor oddity in Sonic 1 that I stumbled across that, once you know how to trigger it, is surprisingly easy to pull off. I discovered it whilest playing the 3DS rerelease about a year or two ago, but have since tested it on a relatively unmodded rom. I was recently reminded of it when I stumbled across this amateur video of someone who had also stumbled upon it (who amusingly doesn't seem to know what debug mode is :Y). The oddity is that you can activate debug mode without entering the cheat. Just beat the final boss, go all the way to the left, and crouch while holding A. When the offscreen Robotnik "cutscene" ends, and your sent to the end game cutscene area, debug mode is active. I have no idea why or how this works, but it almost seems intentional. Doing such an odd specific thing, and getting such a specific result back out, it just doesn't seem like a typical bug. That said, I have no idea why this occurs or if it's even been discovered yet, so feel free to tell me if you know anything about this. I'd really like to know the exact reason why this occurs. I've yet to find anything about this online other then the one video.