Struck through a fairly large, square shaped, struck fragment. The brockage shows lettering and much of the olive branch. Very rare to find this error type on silver dimes, and this is one of the few examples we've ever had.
The fragment probably entered the striking chamber and was struck, then stuck to the obverse die. This planchet would then have been fed into the striking chamber, and the fragment then fell off the die on top of this coin, and was struck creating a brockage.