I hope I don't jinx myself, but that's never happened while quilting. It DOES however, happen periodically when I am piecing. There seems no rhyme or reason for it. I use a little Janome to piece and I never have to mess with the little tension thingy. I have to rip it all out, rethread the machine and practice on some crumbs to make sure it's fixed before I keep going again.
What a pain in the neck.