The most infuriating discussion I had online about proteins was with a vegan, their claim was "there is no such thing as essential amino acids". Couldn't get it into their head that a) there are essential amino acids but b) yes, unless you eat so horribly lopsided it's unknown of anywhere but in horribly deprived populations or among some indigenous folks (pretty much only eating manioc or such) there's nothing to worry about, you'll get your essentials. Kinda like Vitamin C deficiency being unheard of in the developed world because even the most gutter-rat of diets still contains enough as an antioxidant. Still not a bad idea to pair beans with rice and lentils with noodles or bread, though, IMNSHO they just taste better that way around.

Especially infuriating as it was a vegan. If you choose to have a diet that requires nutritional knowledge to get right then don't suck at it, and call your fellow travellers out when they're spewing BS. I really doubt vegans are keen on yet another "I stopped being vegan and it fixed my anaemia" story. Take an apple or two. Either eat them, there's your iron, or make a sauce that works with a sour/sweet accent and prepare it in an iron skillet, there, even more iron. It's not hard but you gotta stop pretending that vegans can get by without understanding nutrition.