Maniac Mansion: Day of the Tentacle

Maniac Mansion: Day of the Tentacle is a point-and-click adventure game released for DOS, Windows, and Mac. As the title suggests, it's a sequel to the original Maniac Mansion, which is included on the disc in some releases.

Unused Item Graphics
There are a couple of item graphics in the game that remain unused.

Noose
A noose. No item of the sort appears in-game.

Diamond
Obviously the one used for the time machine, but you only order it and never actually carry it in your inventory.