Looks to me like the Plane A mappings in VRAM have been corrupted, so: I'd assume it only ocured right there when the text loaded, because of the draw code. Sonic 1 has it's Plane A mappings mapped to C000+ in VRAM, and decompresses the "End results card" art to B000+, and is just barely under 1000 bytes. So, I think, you added that new "SONIC" art onto the end of the text art data, making it larger than 1000 bytes, so of course, when it's decompressed to VRAM B000+ it spills over BFFF into C000+ which overwrites the Plane A mappings (Hence why the top is all scrambled). You need to find a new location for the "SONIC" art. I recommend moving the HUD out of the screen when crossing the sign post, and using that art space for your "SONIC" art (loaded separately).