When using a trackpad instead of a mouse, I often accidentally grab things like files and folders and drag them around. This happens all the time, I guess I'm just sloppy with the trackpad sometimes. Usually, I don't end up losing files or dragging them too far - instead, the files get dropped right back where they were. The correct thing to do here is to recognize that the source folder is the destination folder, and disallow the dragging action. This is what happens in PCManFM. However, there is also a warning box that has to be dismissed. Is that really the best way to handle this common error? In Nautilus, you just hear an exception ding, which is more than sufficient. Actually, I think you could probably get away with doing nothing at all. A dialogue box that has to be acknowledged with a mouse click or a keystroke is just too much to ask for so simple an error.
Obviously, this sort of thing would only be implemented at the developer's convenience.