Neither a bug or Apple madness. Just a damaged database.
Download iPhoto Library Manager and use its rebuild function. (In Library Manager it's the FIle -> Rebuild command)
This will create an entirely new library. It will then copy (or try to) your photos and all the associated metadata and versions to this new Library, and arrange it as close as it can to what you had in the damaged Library. It does this based on information it finds in the iPhoto sharing mechanism - but that means that things not shared won't be there, so no slideshows, books or calendars, for instance - but it should get all your events, albums and keywords, faces and places back.
Because this process creates an entirely new library and leaves your old one untouched, it is non-destructive, and if you're not happy with the results you can simply return to your old one.
Firstly, thank you for helping me out. Secondly, I neglected to mention that this iPhoto library that I'm dealing with is a brand new library that I created from scratch because I was having problems with iPhoto in the first place. I spent the whole day yesterday getting all my photos out of the old library and into the new one. I also spent a lot of time getting all the events, albums and faces right. It's finally back to normal and now you're telling me there's a chance that there's a damaged database. I have to say that what I'm going through is pretty ridiculous. All for a bunch of photos. Everything else seems to be working fine. It's only that dragging and dropping problem that I described. I might just try rebuilding the database using the iPhoto Library First Aid. Thanks again for your help.