Chips in the die. If I found it, I would keep it, but it really doesn't have much extra value beyond the original coin.
Sadly, these types of things on the Lincoln cent is quite common. I have a bag full of 1930's, 40's and 50's with things just as you've pictured. Most commonly found in the 1950's were the "BIE" coins. This is where a die chip formed a blob in between the B and E on Liberty, giving it the appearance of saying BIE at the end. You'll also often find the letters of Liberty filled, as well as filled numbers. Certainly something that is neat, and well worth holding onto. That being said, as for collectables, not very uncommon, but I'm sure there's someone out there in this world that collects these! That would be a neat set now that I think about it! An entire Lincoln series with filled letters/numbers!
