This coin from Brazil KM #556a has some die errors. Coin on the right is a non error coin for reference. The left coin is the error coin. How many different errors are there and what are they called. I know nothing about such errors, so all information is welcome.
@htx, on the reverse of the left hand side coin, a grease-filled die may have contributed to the light strike and missing details (e.g. the star at the top). The front of this coin looks a bit damaged to me and I'm inclined to think that the loss of detail there is due to flattening, which is especially evident on Vargas' nose and forehead.
"The danger we all now face is distinguishing between what is authentic and what is performed." -----King Adz