It happened quite a few times doing normal day to day tasks when I had the pre-production version. The clip was plenty tight, but 2 slick surfaces weren't enough friction for my somewhat thin and smooth pants fabric.
So there is only so much you can ask of a pocket clip, IMO
Saying that it slipped off thin and slippery fabric is kind of like saying my car slipped off the road when it was icy. You just can't (in life) have one thing that covers 100% of all circumstances. Sure, you can get studded tires, but then then you ruin them when it's dry.
A pocket clip is held in place by static friction. The equation for static friction is Fs=us*N (static friction = coefficient of static friction * Normal force). This is a linear equation meaning you can increase the static friction the same amount by either increasing the coefficient (roughness) or the applied force. So, if your slippery AND thin it's basically a worst case scenario. Mathematically you'll have to add a LOT of roughness to see a practical increase in holding force (see my conclusion below). Since the clip is basically a spring, when the spring is "compressed" by slipping it over the material of your pocket, you effectively increase the Normal force and the holding ability of the clip.
Logic would follow that the thicker the material the better the clip will hold, because this increases the Normal force. One of the big problems with older Preons is the texture (roughness) of the body would quickly destroy your pants pocket. I expect this would be even worse with a thin and slippery material. For me it doesn't make sense to ruin a pair of pants through normal use of a pocket tool, and that's why I opted to remove the roughness and trade it for more spring force.
I have a linear force gage and just conducted a quick experiment using both the old Preon with textured body and a pre-production Preon (PVD Black) with the smoothest surface. To draw the light from my pocket, I get 1.8 lbs of force on the old Preon (rough body/old clip) and 1.4 lbs on the Pre-Production Preon (smooth body/new clip). This is only a 23% difference...not an order of magnitude. Next I put the new (stronger) clip on the old preon and got exactly 2 lbs of force. I also tested all of the current production Preons and did not note a statistically significant difference from the PVD finish. That is to say, the difference in coefficient is so small it's not noticeable.
Okay, so yes there is a difference, but does that really matter in practice? I'd say no because you have to think about the physics of what is causing the light to come out of the pocket. It's getting pushed
out, not just falling out on its own. This can happen for a variety of reasons. For example, I carry my wallet in my front right pocket (don't tell anyone) and I've noticed that with some short/fat lights (where the light sits just above the top of my wallet) these get pushed out of my pocket all the time when I sit down, because the light is being pushed by the wallet. It's not the fault of the pocket clip, it's the combination of factors: size/shape of the pocket, pants material, size/shape of my wallet, etc. The force generated in this situation is likely many pounds and no amount of (practical) tightness or roughness is going to keep that light in my pocket under these conditions. The only thing that will have a significant effect is changing that combination of conditions.
These are of course my observations and you are more than welcome to rough up the clip and let us know how you get on. I could be wrong and I'm curious to know. Oh and I want to mention, designing flashlights is really easy as evidenced here.