This one has always been one of my favorite coins.
The Bulgaria 1930 10 Leva coin shows "The Madara Horseman",
"Krum" in the Cyrillic alphabet, and the date "814".

The "horseman" is a rock relief carved into a vertical cliff face depicting a life-size horseman, a lion, and a dog.
It is located in northeast Bulgaria.
Before World War 2, the common belief was that the horseman was Khan Krum who ruled Bulgaria from 804 - 815 AD.
In fact, a Bulgarian coin was minted in the 1930's with a picture of the relief credited to Khan Krum.
Many investigations later, a leading Bulgarian archaeologist, Vesilin Besheliev,
determined the age of the relief at 705 AD, almost 100 years before Khan Krum.
The wrong king got credit on this coin.
I guess this makes it an "error coin".
Bulgaria also struck this coin in 1943:

:)