Struck through dropped filling/dropped letter errors are some of the more dramatic types from the group known as strike-throughs. Their dramatic appearance and unusual method of creation make them a very interesting addition to any error coin collection. They are caused when a design element of the die becomes filled with grease, iron filings, and dust or dirt. These foreign materials usually clog the smaller devices of the die, letters and numbers.
If a coin is struck while elements of the design are clogged, a common error is created known as a struck through grease type. Sometimes, the grease mixture becomes hard and compact through hundreds of strikes, is jarred loose and falls onto the planchet. When the coin is struck, a perfect letter, number or other feature is incuse into the coin.
The dropped letter can fall onto either side of the coin. For instance, even if a number fell off the date on the obverse, it can be struck into the reverse. If you have collected error coins for any length of time, you have probably heard many say that if a coin thought to be an error has an incuse letter, it cannot be an error. That however, as evidenced by dropped letter errors, is actually possible.