I believe you should be getting an extension for a day easily. I know a case of someone on a Vietnamese passport who had the same problem not getting a visa to China with a double entry 2x7 days and he was given a 5 days extension (You need to apply for once in HK).
A one day extension, with a plane ticket booked, will usually be granted at immigration relatively routinely. You will have to pay for the extension--I think it is $150 or so.
However, just showing up at the airport late will be a different story. There you might have to pay for the extension and a fine.
My advice, be polite and apologetic and goto immigration and explain you messed up the counting and you need an extra day.
Thanks for the all feedback guys. I called IMMD in the morning and they said that I should go to one of the immigration offices and get an extension, which should not be an issue apart from having to wait in the que!
I got to the HK airport to fly out and was informed by immigration that I had overstayed my visa by 6 weeks or so! I "live here" (meaning extended stay) but am on a visitor visa and have renewed it a few times. It just completely slipped my mind. I was taken into "the little back room" very politely, was charged $160HK for an extension up to and including that very day, and politely sent on my way. Took only a few minutes.
I now make sure I mark my visa dates on my calendar. Stupid mistake, but dealt with easily. I'm on a Canadian passport and have family in HK, which they did ask me about, so that might have helped.
I had one amusing moment. When being asked about family here, I said "My son-in-law is Hong Kong Chinese..." and got no further before the Immigration official interrupted and said, "He works at Hong Kong Chinese University, is that what you're trying to say?" "No," I said, "I'm trying to say he's Hong Kong Chinese." The man looked very surprised, for some reason. Chinese people in general look very surprised when my daughter says her husband is Chinese. It seems they are far more accustomed to Chinese women marrying Western men but not the other way around. Sorry, that's off topic.
Prepare for the forum attack dogs now Kate, you've revealed you live here on a visitor visa - that gets some of the long term'ers very hot under the collar.