This error occurs during the transfer process and is frequently a result of shrinkage and/or sprocket damage. The result is that the line between one frame and another is visible at the top or bottom of the screen, or, in extreme cases, the film appears to roll between frames.
Can it be fixed?
Minor visibility of the frame line can be adjusted using the crop feature of a video editing program. If the original film is especially shrunken or otherwise damaged, it can be re-transferred using a sprocket-less film scanner.