I called my first ever foot fault a few weeks ago. Guy was serving from the deuce court, inches from the center line and proceeded to throw a poor toss about three feet behind him. The toss carried him well past the center line causing him to hit the ball from the ad side, and he managed to hit it down the tee, which from the angle he hit actually caused it to move away from me.
I managed to get a racquet on it (left a sitter), but called the foot fault. Guy seemed to have no idea that he hit it from where he did. I only called it because A) it was so bad and most certainly allowed him to hit an angle that was physically impossible for him (or me to return from). And B) he called a volley winner of mine out from 40 feet away that was inside the line and didn't even touch it, two points earlier. So I wasn't feeling especially generous about letting that kind of fault go uncalled.