Today, I checked out InkTip's "produced films" page and found this disheartening snippet:
Juan Frausto of Vendetta Pictures has commenced principle photography on the horror feature ‘RoadKill,’ which was written by InkTip scribe David Zagorski...
Even in the film business, I see everyone confuse "principle" for "principal", almost as often as confusing "it's" for "its", but not often enough that I'm sure which version is the correct usage. Google searches inevitably lead me to Paul Brians' amazing grammar cornucopia, Common Errors in English. Indeed:
Generations of teachers have tried to drill this one into students’ heads by reminding them, “The principal is your pal.” Many don’t seem convinced. “Principal” is a noun and adjective referring to someone or something which is highest in rank or importance. (In a loan, the principal is the more substantial part of the money, the interest is—or should be—the lesser.) “Principle” is only a noun, and has to do with law or doctrine: “The workers fought hard for the principle of collective bargaining.” Link.
However, I did hear someone on NPR yesterday say "heart-rending", so I'm not worried about the decline of Western civilization just yet.