Old thread can do this also. I started using my (long gone) Mom's threads and this sort of thing was the result. After the "old thread" incidents I had some problems again with new thread. I talked to my repair man and he told me to use "Bottom Line" -- it is thinner, stronger, and works like a charm. If I have trouble now its because my machine has been moved and the settings are whacky. Luck of the Irish... I walked into a store and they were discontinuing their Bottom Line and I bought about 30 colors for half price!!!!