When you perform a glitch that accesses memory it's not supposed to, a common effect is to overwrite the graphics of the game (VRAM).
Light VRAM corruption
Light VRAM corruption will overwrite a small number of pixels, leaving the game just as playable. You can fix it by mirroring, or with a save and quit.
Heavy VRAM corruption
Heavy VRAM corruption makes serious changes to the game's graphics, leaving the screen mostly incomprehensible. Because you can still see patterns of 8x8 tiles, it's possible to do something familiar like fight a boss in this state -- and in some glitches, you will need to.
After you've accomplished what you need to do, save and quit and hard-reset the game to get the correct graphics back.