I added only a few more PPU writes during VBlank and then I got an unexpected error: my stack blew up with repeated numbers and this ultimately is very bad. It didn't do the screen flicker that happens when you write to the PPU outside of VBlank but rather wrecked my stack. I believe the extra PPU writes were the cause of this because I deleted different PPU writes nearby that were working just fine and then the stack explosion didn't happen so I diagnosed the problem as too many PPU writes during VBlank. Why does this happen?