I should add that the HDS (and NovaTac) is protected against button "clicks" that are too short, probably to prevent activation due to bumps etc. If you're fast on your fingers and make a really fast click it wont register. This makes the problem I describe very unlikely to turn the light on unless the spring ends up in permanent contact with the retaining ring, as it happened for me once.
The protection against too short presses ironically is not in effect when the button is locked, so when banging the light against your palm you get the two warning flashes each time.
So, if you want to check out if your tailcap modded HDS has this problem, activate the button lock faeture and lock the light. Then try banging the tailend moderately hard against your palm and see if you get the warning flashes.