Out-of-bounds SRAM Editing
The EG2 map is an underworld map of single-entrance rooms. Instead of the exit doors of these rooms having a specific destination, they will simply return you to the overworld location where you entered them.
Most of the addressable area of this map is unused. If you use glitches that copy SRAM (save RAM) data to adjacent tiles while in an EG2 tile, you can affect the SRAM addresses corresponding to the unused rooms. This is past the end of the "room data" part of the SRAM map, so instead it will change arbitrary other properties in your save file.
This demonstration video by Glan shows SRAM editing that causes the save to have all 7 crystals, among other effects.
This technique is banned in all race rulesets (and is even banned in the "Unrestricted" ruleset of vanilla speedrunning).