Sport yarn will break when trying to unravel or frog it.
I only ever had that happen once, with a lace weight cotton blend yarn, Noro Taiyo. I had to be extremely careful casting on and off as it broke so easily. I've never had it happen with other fine weight yarns as they generally are more tightly twisted and therefore stronger.Sport yarn will break when trying to unravel or frog it.
I had one of those! The “environmental conditions” seemed to have been marauding moth larvae, though I never saw any vacant casings. Just frequent spots where the yarn just fell apart at a touch. At a guess, it was 100% wool very fine fingering yarn designed for fine blouses or infant layettes … pre-WWII. I used it anyway, to knit large, loosely knit mitred squares, which were then unceremoniously tossed in with the laundry to felt. Ravelry: JessicaJean's Mitred Divided Squares - felted and not yetIt isn't the fact that it is a "sport" weight yarn, it is the brand or fiber of the yarn that is the problem. It could be a "cheap" brand, or a weak fiber, or even older yarn that may have been exposed to environmental conditions that weakened it.