It may have been a knot tied at the factory in skeining up the yarn. Usually people cut them out and join in another way. I test them - if they slip a little I'll cut and reknot (because my knots don't come undone and I anchor them so they don't move), if they don't, I leave them. I don't like knitting the ends overlapped, I think it's too bulky, even with thinner yarn and a loose gauge.
sue- knitting heretic