My wife's Singer 500 was purchased in 1961 and she stated that this has never happened to her while sewing. It has happened when starting if she does not hold the threads. She also mentioned to check the entire thread path, top and bottom and try another bobbin. This will happen if a bobbin is wound to loose or to full. . If her 500 acts strange in any way she performs the complete maintenance that was shown her when she purchased the machine.