There is a known story about Rav Mordechai Schwab Zatz”l of Monsey. He was once walking to Yeshiva with two talmidim, one of who would walk with him often and one who it was his first time. When they past a certain point on the way, Rav Schwab,said With his usual smile and warm expression,”Good morning”. Not noticing anyone there,the newcomer quietly asked the usual one, “To whom did the Mashgiach just say good morning??”
He answered, “There is usualy an elderly woman standing at that spot waiting for someone, at the time the Mashgiach passes there. He made it a practice to say good morning to her every day, but since he was makpid on “Shmiras Haeinayim” he did not notice that today she was not there!, so he just said his usual warm heart-felt Good morning, when he reached the spot where she usualy waits, and continued on his way!!!