For posterity the problem was that I had a shared swap partition which of course didn't show in the list of mounted stuff. Added it a week or two ago and forgot it.
But my original point was that this was an editor which lost what was being edited just because it couldn't save. With no warning. Would anyone tolerate that in a word processor? I had about 1/2 hour into that session, copying and pasting, changing between starting and ending points and starting points and lengths. Math/calc was really handy.
