When I moved my iPhoto library across, all seemed fine ... except that about 80 of my video clips appeared with the keyword Holiday. I was pretty certain I'd never coded them in that manner, and sure enough, a glance at my list of keywords showed that Holiday wasn't in the list. Now, it's entirely possible I added it at one point, then removed it and forgot about it. In any event, the question was how to get rid of the unwanted keyword on my movie clips. I couldn't just delete it, since Holiday didn't appear in the keyword list (and yet, Smart Albums using a select where keyword contains Holiday worked perfectly).
The solution? I had to use the Keywords tab in iPhoto's Preferences to add the keyword Holiday. Once the word was added to the list, I was then able to delete it from the selected videos without any troubles (and I then deleted the keyword when I was done).
I have no idea how this happened, but if you see something similar, at least you'll know what to do.

