In a certain situation, some images temporarily won't appear on the light table.
If the light table is selected in the project list, and you import images while it is selected, those images won't be eligible for the light table until you first visit a project or the library. They will appear in the browser, but not on the light table when you try to drag them there.
To see the images, simply click Library in the projects list, then go back to the affected light table.
This issue is resolved in Aperture 1.5.