Not so much a thick passenger comment but just 'thick passengers'.
Opposite the CSD office is a toilet and I reckon 90% of passengers try and open the door with the, now defunct but still there, ashtray I have watched them pull it push it whilst ignoring the actual door handle until they are holding the ashtray in their hand.
They usually get Arsey as if it is your fault that they are too thick to recognise an ashtray as opposed to a door handle!
This particular toilet is for use of business class passengers so not exactly folks unfamiliar with aircraft facilities.