You should make a habit of checking your equipment before and after each use. I've been using a Lee holder for several years and have never had any problems with it...then again, I always give my gear a once over before using.
Something else you need to be careful/aware of: the spring in the brass clip has a fairly low spring constant, which makes the spring force a bit "soft". If you happen to be handholding the camera with the holder installed, and the holder should "catch" on something, it will separate from the adapter ring (rather than being held firmly and the "catching" force being translated to the lens mount)...this will protect the camera/lens, albeit at the cost of a possible lost holder and filter. So if you happen to be resting the camera/holder on a railing or something similar for support, be mindful once you've taken the shot and lift your camera.