If you have a lot of folders nested in a folder, and you are dragging files over the collapsed folder... if you pause too long, the folder will "spring open". This is great, actually. But the problem is, if the "opened" branch contains lots of items, the window will scroll, MOVING the folder you were trying to drop files into. If you aren't paying attention, or your timing is just off, you'll end up releasing the mouse button and dropping the files into a completely unintended folder.
When "springing open", the drop-target... the folder you're dropping the files on to... should NOT MOVE. This is very simple usability. Unexpected things like your drop target moving out from under you unexpectedly should just never happen.
The solution is simple... though its implementation might be tricky given current APIs on the controls involved, I don't know... but it's to have Windows Explorer recognize exactly where the current drop target is in the window, and ensure that when it springs open the folder you're hovering over, that the position of the folder does not move with respect to the window.