I believe some collectors call it a "stuck digit", like an old style gas station pump. Your example is very minor.
Sorry to be the bear of bad news but I come across those daily. At first I had too thought that they might be some kind of error but after seeing too many, I started to find out for myself that it is not extreme enough.
It might be a start for a fun set of increasingly stuck digits, that might be the second in a series, the first could be normal.
Thanks for the replies. I am still learning about FRN. Are there any good resources on the web to learn more about all the numbers and letters on notes and another info that I need to know to become a more well rounded collector? I do have the United States Paper Money from Red Book.