This is my first post on the website and hope it gets out. I have just watched an episode on TV and saw that the figure-8 knot shown was tied completely wrong. The knot on TV was only half a figure-8 knot, so of course it slipped. From previous articles in magazines I remember the improved clinch(trilene) knot is about 90-95% the strength of the line. The figure-8 and palomar knots are 95-99%.
The way you tie the figure-8 knot is too make the knot like shown on TV except the eye of the hook goes on the tail of the shown knot, then back up to the already 1/2 figure-8 and the line follows the line back through the first 1/2 of the loop.
Here is a link that shows how to properly tie the figure-8 knot..
Hope this helps.