Degrees of uniqueness
A special mold is the most expansive form of uniqueness that involves a lot of investment by the LEGO company. Really unique molds are rare, but they occur, e.g., in licenced sets, as minifigure parts or accessories. For a long time, the One Ring from one LoTR LEGO set was unique, until recently revied as a wedding ring in the Brickheads series.
This kind of uniqueness is somewhat more frequent: You can have known element in a unique or rare colour for some model.
Even more frequent are unique prints on known elements of known colours, including the bodies, legs, and heads of minifigures, or special printings bricks.
Stickers from sticker sets are really often unique. It is not unusual for them to contain a sticker with the official set number.