My customer was upset that she found a knot in her ball of yarn. Of course she was! I remember the first time this happened to me–I think I was about 10-11 years old and I was outraged!
But after a lifetime of knitting/crocheting, and more than 10 years owning our business, I have learned a lot from manufacturers., Of course, they cannot intercept every flaw while putting up the balls of yarn. So some will always get through; the standard for knots is 1) no more than one per ball, and 2) a knot in less than 3% of all balls. So this is a very small probability of getting a knot, but at the same time most knitters have experienced this at least once. I have had manufacturers recall yarns because of knots as a defect, so I know they do keep track of this as a quality issue.
This is how I deal with knots that occur in the ball: If it happens near the first part of the row, that’s great. Just unknit back to the beginning of the row and then start that row after the knot, leaving enough extra on the edge to weave in. If you don’t want to do that (let’s say you’re knitting a 48″ wide afghan and you have gone across most of the row) , you can weave the ends in within the fabric by allowing about 6″ on either side of the knot. I usually take about 6″ on either side of the knot and create a slip knot with those tails very close to the needles. When you’re finished with the piece and ready to do your sewing, pull out the slip knot, clip off the manufactured knot, and weave in the ends.
I know it’s annoying, but it is unavoidable over a lifetime of knitting. I hope this helps.