I am one of those natural-born editor types, so I am very particular about how words are used. I’m sure most of my readers do this to a small extent: if, for example, you were to read the sentence “He is two old too do that,” it would probably drive you crazy. I’m the same way, but about zillions of words.
A big pet peeve of mine, having to do with chemists: misuse of “principle” vs. “principal”.
Principle: An idea. For example, the theory of relativity was a principle that radically changed physics.
Principal: Main. For example, the principal wage-earner in our house is my husband. (This spelling also refers to the administrative head of a school, but that’s irrelevant to the discussion.)
It’s amazing how frequently these are misused by chemists: I’ve even seen it done in textbooks. There are many principles (ideas) in chemistry, but, generally speaking, the word that is used in my science is principal (main):
Principal quantum number–the most important quantum number. I’ve seen at least two texts spell it as “principle quantum number” though. Sorry, folks. A quantum number is an idea, but that’s not what we’re trying to describe here.
Principal investigator–in other words, the main investigator/researcher on a project. If you can’t remember the right spelling, just call this person the PI, okay?
…and while we’re on the topics of pet peeves, if I ever catch you spelling words as “flourine” or “flourescent” I will whack you over the head with a foam bat.