I had similar experiences with Delta's web page some time ago, when it was pretty new, but not Hilton.

I did have a Hilton web site problem a couple of years ago that might be related, though. It happened at the time we could make a reservation online by simply indicating our HHonors number, without a password, and providing a credit card. Evidently, someone in my office erroneously used my HHonors number, and when making the reservation and changed the name and address. When I discovered it, it took quite a bit of convincing (and a faxed copy of my Diamond card showing my name) to get it my account back.

I presumed that the recent requirement to use passwords was to prevent identity theft problems, but maybe they still have some bugs to work out.