Hi Kevin- and welcome to BPN as well.
There's been one thread I know of on BPN regarding the top plate coming off the legs of Gitzo tripods, here:
http://www.birdphotographers.net/for...ht=gitzo+plate
The problem is well known enough that a commercial solution is available here:
http://www.naturescapes.net/docs/ind...richard-peters
I came up with my own solution here:
http://www.birdphotographers.net/for...ad.php?t=64125
When I received my model 3531 Gitzo tripod I noticed right away that there is no fail-safe for the top plate falling away from the Systematic tripods if the pinch bolt is not tight (I'm a motorcycle restorer in my spare time and my motto is "if it can go wrong, it will go wrong"- this is AKA "Sod's Law"). Gitzo makes a lot out of the Safe Lock system on the top plate and there's even a little grub screw to tighten down on the underside of the head to stop it unscrewing, but there is no mechanism to prevent the top plate from coming away from the tripod legs, except for the pinch bolt. I can think of several ways of engineering a solution that is much more elegant than mine or the commercial one so I am confident it will happen sometime in the future.
There is an analogous example in the area of Arca-Swiss plates and clamps. If you don't tighten the clamp, the plate can slide out, so guess what, almost all plates now come with two little safety screws to stop a catastrophic slide in the event that "Sods's Law" takes effect. I doubt whether plates have slid out too often in the past but those two little screws give you a whole lot of peace of mind in any case.