For me it is the fact that our blood contains iron. I earlier used to believe the word stood for some ‘organic element’ since I couldn’t accept we had metal flowing through our supposed carbon-based bodies, till I realized that is where the taste and smell of blood comes from.
Years upon years of being told this cannot make me not taste metal from stainless steel cups/canteens and forks, even brand new and/or freshly scrubbed to hell and back. I can’t use stainless steel tumblers because of this - even if I keep my tongue well away from it, and it’s the cleanest dish in the world, it makes the drink taste metallic. No amount of youtubers just insisting I don’t/can’t taste a thing can actually compete with a lifetime of experiencing this problem. And I have, multiple times, tried all the things they say to do to fix the “real” problem - but no. Steel tastes like steel, always.
Hypothesis: this is one of those things some people can taste and others can’t, like how there’s a whole group of “cilantro tastes like soap” people and everyone else is like ???