Back in the days of London Airways, one day in very bad fog conditions the D Controller displays were full to overflowing with Flight Progress Strips and a lot of high level holding was going on. Somebody inadvertently knocked an FPS off the Clacton designator and it fell on the floor behind the display. Sometime later when traffic got moving the pilot called to ask how much longer he would be holding and it suddenly dawned that traffic had been descending through his level in the hold for some time!! This was in the good old days when radar was subsidiary to the D Controller and there was no SSR.