I want to preface this by saying that I was raised on the "no knots in knitting (or crocheting) EVER" philosophy. Having said that, I have been having problems with weaving in ends on the slippery cotton/acrylic blend I am currently using and ran across this video
and went "Hmmmm...wonder if this would work". I've used some scraps to try it out on and the knot seems to hold against pulling using all my strength. It does leave a very slight lump...with my yarn the double-knot is about the size of a #2 pencil lead...but balancing that against not being able to keep those danged slippery ends woven in, I'm wondering if the trade-off may be worth it. My greatest concern with this knot technique is that it may, sometime down the road after several washings, come lose. Again, my tests seem to show that it is lock-solid but I thought I'd see if anyone has any first-hand experience with this specific knot's stability.