A 2005-P West Virginia state quarter has been found with a “dropped letter” showing as a letter “T” dropped out in the field. This is the result of the “T” of WEST being clogged with debris that was packed in so tight that when it eventually fell out of the die cavity of the “T”, like Jello from a mold, it was struck into the field of the coin leaving behind this very interesting incuse “T.”
Another find is a 1988-P Jefferson five-cent piece that is described as an in-collar flip-over double strike. It was struck normally on the first strike and then re-entered the collar flipped over and rotated in relation to the first strike and struck within the collar again. Because the coin expands ever so slightly after it is ejected from the collar, it is very difficult for it to completely re-enter the collar. As a result the most in-collar double strikes will show what is called a “Partial Collar Strike”.
Resource: Numismatic News Oct. 14, 2008.