Minecraft v1.16.5
Forge 36.0.1
Quark r2.4-295

If the magnet system config is modified to include modded items, but said mod is removed, the game will crash instead of ignoring the item id. For example, while building a modpack, I originally had iceandfire:copper_ingot in the magnet derivation config, but then removed the mod and got a crash.
Crash Report