At the risk of being a cynic, well-tied knots don't fail. As a volunteer instructor at the Michigan Trout Unlimited School each year, I have studied and taught fly fishing knots a bunch.
Look at the failed knot. If the remaining end is a curly pigtail, then the knot failed. If it is a clean end, then the tippet broke.
When tightening up the knot, wet it to provide lubrication, and draw it up slowly to reduce friction and resulting heat, which weakens monofilament.
Light tippets can be protected with light rods and soft strikes
BTW, anyone interested in the school to learn or improve your fly fishing skills. (June 11/12/13 this year) please visit
www.tuffs.org