Corrected entry: The robot Gerty that appears throughout the movie has a small shelf on its right hand side (viewers left hand side, robots right hand side) on this shelf sits a dirty coffee cup but all through the movie the cup disappears and reappears several times.
Dandaire
4th Nov 2009
Correction: The film takes place over a long period of time. It's reasonable to assume that the various Sams have taken coffees, or that Gerty tries to keep a fresh cup there, should Sam ever want one, but various tasks interrupt that.