Comment 7 for bug 1029371

I have also noticed that this does not only happen when dragging and dropping among and within Nautilus windows and/or the Desktop, but also when dragging a file to an application capable of handling it, such as dropping a file into Filezilla (on the server-side panel, i.e. the right one) for upload. In that case, also, at random times the operation is silently ignored.

I would say it does appear to be more likely to happen when the drag-n-drop is done quickly, but it's difficult to say, because most of the times I do it quickly, unless I am re-doing it just after a failure.