At first I thought acid corrosion ,but that doesn't seem to fit. Notice the indentations and that the original surface metal is still present, the inner zink is not exposed .Therefore it Can't be acid. Then I thought a very clogged die, But there is those horizontal indentations again. I think I know what really happened here but I want everyone to have a chance at a guess before I give my opinion. Good Luck!
This is a capped die strike. The coin was struck through an obverse die cap. The cap had a wrinkled surface, which is pretty common. The cap was thin enough by the time this coin was struck for a ghost of the obverse design to bleed through.
Its obvious you know what you are talking about. What did you mean that it is "common"? I thought these were fairly uncommon.
Depends what your baseline is. Compared to the total number of coins struck, these errors are rare. Compared to other types of errors, capped die strikes are fairly common. They sell for about $10 on eBay.
