The hospital doctor says the effect of the invisibility drug will die along with Dr. Griffin (the invisible man). Since the invisibility drug is unknown to anyone, there's no way for him to make such a "knowledgeable" statement.
One of the tricks used to suggest Griffin's invisibility was the simple use of black cloth to hide his exposed flesh. This is particularly evident when Mrs. Hall barges in on the Invisible Man while he's eating. The black cloths covering the lower part of his face and his wrists as he holds the serviette up are clearly visible.
In the last scene in the Lion's Head bar, the Invisible Man lifts a bench to kill Inspector Bird. The bench was actually lifted by wires, and the technician(s) who operated them apparently misjudged the distance when they brought it down on the prostrate man. It appears to hit him on the nose - hard enough for him to reach for the injured proboscis before dropping his hand as he "dies".