When the author sees someone wearing improperly adjusted glasses, he recognizes that somewhere an optician has been trained improperly.
The most common error I see is temples that are fitted too closely to the patient's head. This has two consequences. First, it will create a deep indentation in the side of the patient's head that not only can be clearly seen but felt. Often the indentation is so deep that it is clearly visible after the patient takes off the glasses.
Second, the inward pressure of the temples against the side of the head will make the glasses tend to slide forward. This will cause the patient to push the glasses constantly up each time they creep down his or her nose.
One of the keys to learning to fit eyeglasses properly is to understand the fitting triangle. This concept is described brilliantly by Russell Stimson in his book.1
The availability of frames with spring hinges seems to have made matters worse. Inexperienced and ill-trained opticians rely on spring hinges to hold poorly fitted eyeglasses in place. The springs only work, however, when the frame is adjusted tightly enough for them to flex. This increases the inward pressure of the temples against the head. When these temples are adjusted incorrectly, the springs will squeeze the glasses forward forcefully and continuously.