I was reading a book today. I know, shocker! I won't write the name of the book, but it is a fairly well know children's series. I found a grammatical error. Horrors! This makes me sad. I expect to find errors in self-published e-books; but not in an edited, printed, professionally-published book! The author used the word principal when he meant principle.
A principal is the person who runs a school.
A principle is a tenet or rule that one follows.
The principal is your pal (a person), as someone once taught me.
I have frequently seen these words mixed up by people writing comments on the internet. It's just another one of those stupid homonyms that people don't know how to use, or don't pay attention to. Why do we have so many of those (homonyms, not people)?