I personally would not want the embarassment of telling me there's a booger on my nose, so I would refrain from telling that person. Besides, I find it hard to believe that one can have a booger on their nose and not quite know it. I don't like the feeling of boogers in my nose, so I almost always know if there is one hanging. Now how about telling a man (woman) that his (her) fly (slip) is open (showing)?
Chuck