Give/record helpful feedback if a drag & drop action fails
Use cases
When dragging and dropping a file from Nautilus into another program, if the action can't be completed then the icon moves back to the Nautilus window with no further feedback regarding why. It could just be simple user error, or it could be a more complex issue involving permissions, or either app, or their libraries...
Examples: dragging an image file into GIMP, to either open it or add it as a new layer, but it won't go; dragging an image into MusicBrainz Picard, to update a resource's cover art, but it won't go and there's no other mechanism to add it.
Desired behavior
It's probably not a good idea to display a modal dialog with the relevant information for every failure, especially in the case of simple user error ("Why are you dragging a .jpg into Sound Converter? Don't do that. [OK]").
It would be really helpful if, at least, the failure and the text of any corresponding errors were logged somewhere for further investigation. Taking it further, maybe there could be an error-reporting widget in the top of the Nautilus window, temporarily displayed like the transient file-copying widget, which similarly expands to display recent pertinent information or open the log when selected?
Benefits of the solution
Users would get a better idea of which drag & drop operations are appropriate for their system, and which aren't.
Users could also use the logged information to search and further diagnose changes in behaviour which worked before, but don't anymore due to updates or conflicts or other differences.
Possible drawbacks
If the user is a prolific dragger, this could fill up the logs with a lot of cruft.
An extra widget at the top of the Nautilus window could get annoying if it happens a lot, potentially leading to users frustrated with it and wanting to get rid of it.