So I've ran into a couple of problems whilst emulating Sonic 1. First is that Debug mode won't allow me to place any number of items over a certain amount (ex. can't place more than 70 rings), it also places destroyed monitors instead of new ones if I break any monitor in a level. Second is that when using debug mode, the game won't allow me to go past Labyrinth Zone. Once I complete that zone, it takes me back to the title screen. Are these normal? Is there anyway I can fix them? Do I need special editing tools? Maybe it's the ROM I'm using? Help!
The issue with placed monitors being broken is a standard issue with debug mode. It effects Sonic 2 as well, and you need to modify the code to fix it. Not being able to place unlimited objects is normal as well, the game can only run so many objects at once before it runs out of space for them. This game runs on a console from the 80s with limited ram, you can't expect it to support infinite objects at once. I've never heard of that labyrinth zone issue before, however. Which emulator are you using? Are you sure the rom your using is a vanilla copy of Sonic 1? That issue doesn't sound like something that'd be caused by a bad emulator, but I've been surprised before.
I have yet to try it on a vanilla Sonic ROM, but this problem was encountered while playing Cinossu's "An Ordinary Sonic ROM Hack", mainly because that game is borderline (if not) impossible to beat without some aspect of cheating. I'm also using Picodrive Genesis emulator for the PSP, if that means anything.
I'll bet that's entirely intentional then; it was likely coded in to prevent cheesing your way through the game. Cinossu doesn't want you doing that, obviously :Y
The Genesis is a very limited system, and can only display up to 80 sprites (Cells, or mappings if you prefer) at once with 20 on 1 scanline. Also need to put in consideration the limited Sprite Status Table size. Ignoring the reserved table for things like Sonic and the title cards, the dynamic objects go from $FFFFD800 to $FFFFEFFF, with each object being $40 bytes long. Meaning, you can have up to 95 objects (Feel free to correct me if I'm wrong) So, chances are, the sprites are there, but there's too many graphical sprites on screen.
It was not an anti-cheat measure. You simply can't access Star Light and Scrap Brain without using Sonic 1's Level Select (or game genie codes) regardless of whether you've been using Debug mode or not because they were never finished in that hack. The Labyrinth Zone issue that was described in the first post is completely unrelated to Sonic 1's Debug mode, and will not happen in vanilla Sonic 1.