Knots in the yarn
I try very hard not to sell yarns that contain knots, especially in superwash yarns as the yarn is trickier to join together neatly. Generally these are sold as seconds and the listing will state that there is a knotted join. Yarns that have not been superwash treated are allowed to pass my "no knots in the skein" rule as long as it is only the one knotted join. Non superwash wool can easy be spit-spliced and felted. More often than not, yarn is joined together in the mills in a splicing method which is perfectly strong enough to knit through. Please bear in mind that my skein winding equipment is motorised so it is more difficult for me to always catch knots.
More often than not at the mill, yarn ends are thermally joined together, so fused not knotted. The fuses do not show once knitted though.