The line on the right creates a "knot" in the line, and doesn't look smooth. This can be mitigated by using a different colour for the extra pixels, creating a smoothing anti-aliasing (AA) effect. Some artists also take advantage of the "knotty" look by using it for actual features in the image, such as areas where the line literally widens, or where other lines connect to the line. In many cases, however, that look is just a mistake on the artist's part.