There are those who seem to feel that it is important for them to ruin a really great meal in some way or another. Here are a few of the ones that irk me the most. Please feel free to add to the list.
1) Bring up someone's medical issues.
2) Make a remark about how some of the food items, ingredients, or method of raising is detrimental to health.
3) When the meal is over, declare that after that, no one should have to eat again until some (large) amount of time has passed.