For example:
Carnist: "Hey, you want a slice of this pepperoni, triple cheese pizza?"
Vegan: "Sorry, no thank you, I'm vegan."
Carnist: "Oh, wow! Nice! I respect that! Yeah, I would go vegan, but I just would struggle with protein, B12, and iron if I did! Like, I tried being vegan for a week and got so sick! I still eat plant-based alternatives every now and then, though! And I totally get where you're coming from with your ethics! Yeah, the way they treat animals on those factory farms is crazy, bro! I hope those cruel practices can be abolished one day. Complete solidarity with you and the animals, dude! ✊ In the meantime, I absolutely try my best to get my meat, dairy, and eggs from local, free-range, humane, and organic farms!"
Vegan: "Okay?"
Like, seriously, just saying "Oh, okay." and moving on with your life would be preferable over this shit.
It's a real tell about their own feelings and cognitive dissonance. Just existing as a vegan is enough to make people defensive, because by existing as a vegan you're showing that being vegan is possible. This shows people that they don't have to be aboard the carnist treat train, but they are.