If you have a File Collection that contains a few files and add a few sub-collections to it, the files that were originally in it can't be dragged to the new subcollections (although the cursor reacts as if the operation will work and even changes to the one with the "+" sign if you press Ctrl).
What makes me think it's a bug is that if I open one of the new sub-collections in the next tab and drag and drop a file from the original parent collection to the tab title, the file actually moves (or gets copied) to the new sub-collection.
Nothing horrible really (I just work around it by opening a new tab) but if it can be made to work without an additional tab I'd be grateful 