two or three

If you say "there are 2 types of people, those who do X and those who don't do X",
then you have actually split the world into 3 types of people - those who do X, those who don't do X, and those who don't fall into this dichotomy.

Also :
"When making a peanut butter and jelly sandwich you must first apply the peanut butter, not the jelly to the bottom half of the bread."