What is the most common cause of foreshortening (teeth appearing shorter than actual) on a periapical radiograph?

In periapical radiography using the bisecting angle technique, foreshortening occurs when the X-ray beam is directed at too steep (excessive) a vertical angle relative to the bisecting plane of the angle formed between the long axis of the tooth and the film/sensor. This causes the image to appear shorter than the actual tooth. In contrast, elongation (teeth appearing longer than actual) occurs when the vertical angulation is insufficient (too flat). Horizontal angulation errors cause overlapping of interproximal contacts. The paralleling technique minimizes these errors by using film holders that position the film parallel to the long axis of the tooth.