Yet it's still very common to run into people that 
    are absolutely convinced they have a sensitivity to 
    MSG despite the fact that all evidence suggests that 
    no such sensitivity actually exists. 

Here's a pattern I've seen in at least two people here in America. I suspect it's probably pretty widespread.

1. Subject is a person who does not eat Asian/Asian-American food very often

2. Subject eats some Asian/Asian-American food, full of many ingredients they don't typically consume

3. Food does not agree with subject in some way or another

4. Subject then decides it "must be the MSG" when of course it might be just be high sodium content that is common in takeout/delivery food, or any of the other ingredients they don't normally consume, or perhaps they just got some spoiled food or something