Meow bought a Jefferson Nickel coin cover on fleabay. Meow was not expecting the Nickel on the cover to be so interesting. It was a bit of a surprise. A nice example of grease on die nickel.
This reminds me of a similar feature frequently seen on ATB Quarters. Mike Diamond says this is a "struck-through" error in which the foreign material on the die face was constantly rearranging itself through the press run. Ken Potter agrees and says offending material like this can stay within a pair of dies shifting around for hundreds of strikes at times. The nickels on the First day of Issue Cover is neat!
Meow is pretty sure this is a instance of grease on dies. Very common on ATBs. Meow has a small 2x2 box full of flips with examples of that.
Especially with the low relief of current dies... It's not getting stuck deep in the die to hang around for a while