From a sometime programmer's point of view, I think what's going on is that the memory buffer used to store the list of items that's used for drag and drop operations is running out of space. (aka there are too many items to fit) and so it cuts the list off somewhere and the last item gets assigned a null kuid. (aka kuid:0:0 )
For anyone who didn't see this before, if you right click and choose Download the problem won't occur and all the items should get added. (Well, unless one of those items is a built-in item with an update available, that's a special case.)
Edit: Oh, and this isn't mentioned in the list of SP2 fixes as far as I know. There's also another Download Helper bug (at least in SP1) where operations intended to be done on an item show the wrong item, because there's an item above it with dependencies that is collapsed. (aka not showing the dependencies) Workaround there is to make sure to expand all the items with dependencies. (aka click the triangles)