While browsing a number of Gallic Celtic coins, I noticed that the Bellovaci kneeling man bronze formed a hidden face on the obverse.
In this example the lighting of the image causes this to be obvious (image used with permission of Marc Breitsprecher)
Obverse: Kneeling man right. Eyes formed by elbows and brow by shoulders, the nose by the back and buttocks, and the mouth by his legs.
Reverse: Bristled boar left
AE 19 mm; 2-3 gm; BN 7309, SCH/D 311, Sch/GB 163; class IIIc; 80-50 BCE