Knitters are dismayed to unexpectedly find a knot in the yarn they are working with. Knots are often hidden deep within the skein and are not revealed until the yarn is pulled out. There are ways in which a knitter can disguise the joining of the ends of two separate lengths of yarn -- if the break is known ahead of time.
I am told by my LYS -- local yarn store -- that makers of yarn are allowed a certain number of knots within a skein. Only one or two. I recently found some long-forgotten green Plymouth Yarn -- Encore -- in a file drawer I obviously hadn't opened for a long time. Perfect for knitting a cap on circular needles, with very few places in which to hide a yarn joining.
Oooops, pulling the yarn out from the center of the skein, my fingers discovered a knot. Curses on the Plymouth Yarn people. What to do? Okay, I switched from a knit stitch to a purl stitch. Any interruption from a straight stockinette stitch would help hide the joining, and I could make a couple of rows of purling look like a design. Purling away, I felt the knot go through my fingers. Three or four stitches along, I stopped to view whether the knot had fallen on the inside -- hopefully -- or on the outside. I could see no knot, but something was wrong. So, I unknit for a few stitches, and the knot became undone.
Sigh of relief. The knot was not the tying together of two loose ends, but truly a knot that under the right kind of pressure would undo. Take back my angry feelings about Plymouth Yarn folks.
Precaution: Still haven't finished the cap -- there may still be hidden breaks ahead. I'll let you know.