Turns out, it clears with either a restart or a logout-login turnaround; either one should work.
In my opinion, though, any failed or cancelled copy, move or download attempt should clear from the queue right away. Now in cases like the one in my OP, it's almost forgivable that Nautilus would hold on to that task in the File Operations window, as it could not
confirm that the "from", "to" and "what to copy" were no longer available (it was N
that lost the FTP connection, btw, as the copy was started).
Still a little buggy. Probably see a fix just before they sundown GNOME2 for good.