This happened to me last month.

I only noticed because my AAdvantage account showed that 18K miles that I transferred in from my HHonors account had been returned to Hilton. This meant I didn't have the miles I needed to book an upgrade for an upcoming flight.

When I called Hilton, they discovered that they had "accidentally" merged my account with that of someone with an idential name in the US (I live in the UK). We shared an Honors a/c # and PIN #.

99% of the stays and points on the account were mine, but they still cancelled my account and transferred my miles into a new GOld account. I asked them to re-transfer the points to my AA account, but they still haven't posted there.

And what do I get for all this confusion and inconvenience, plus the likelihood that I won't get the upgrade I wanted on AA? A lousy 3,000 HHonors points.