I am mostly very happy with my David Sorg easel, but from time to time I have a problem with the support system and panels. The easel uses coarse grit sandpaper attached to flat boards top and bottom to hold the canvas, which is nice because you don't have flanges covering the art or overhangs casting annoying shadows. However, with 1/8" panels, there is less surface area to grip, and sometimes when I am getting enthusiastic with the palette knife it will come loose with an alarming crash.
This seems to be more of an issue with the top bracket, and with a larger panel, probably because it creates a long lever amplifying the force against it, and because the weight pulls it away from the sandpaper.
My solution is the Scabbet (scab + rabbet), a temporary backstop attached with Velcro. It makes a significant difference to the way it feels to paint, much more solid and dependable.
David Sorg's solution is simpler: http://www.studioeasel.com/tune.html