I see it very often with Coats & Clark cotton machine quilting thread & just recently their All Purpose thread. When I first started quilting, I had a spool of some no-name stuff that slubbed & broke constantly while handsewing.
I have also had some of the C&C variegated machine quilting thread that the thickness of the thread changed drastically at some of the color change areas causing my auto tension sewing machine to throw hissy fits.