For example there's a wizard-type enemy that temporarily debuffs your max health by 50%, and the effect can stack if you get hit more than once. But what happens if you pick up a permanent health boost item while the debuff spell is on you? You gain the normal health boost, then it multiplies itself when the wizard's spell expires; you can can go from 100 to several thousand HP in seconds and there's basically no limit.
In any other game this would be considered a bug and patched out in the next update. NOT NOITA, motherfucker. You're almost required to do things like this for a lot of the end-game optional content.