Revealing mistake: Initially, the gallon plastic water bottle Joey was bringing to Hamby looked almost full. However, the ease at which he raised it into the air (by his head), "hollow" sound made when Matt took it from him, and the way it tilted to the side when being dropped down the hole are indications that the water (and its weight) had been removed for most of the scene. When Joey dropped a plastic bag containing a bologna sandwich, chips, and a soda pop down the hole, it went straight down without tilting.
Revealing mistake: From above, the hole did not look very large (maybe 3' in diameter); it was about 20' deep. It was obvious various modifications had to be made to film in a way that would enable Hamby to be visible. Sometimes, the "sides" looked flat, not like a tunnel or long tube. In front views of Hamby, the diameter was increased, giving him more room to move. When Joey was talking to Hamby, a shot from Hamby's perspective just showed Joey's face as though it were setting directly on the edge (or grass).
