What is this called?

The zinc core of the coin is oxidizing/ corroding underneath the copper plating.
Sometimes the zinc planchet can get and retain contamination before it is plated.
The plating seals the contamination in. The contamination will expand and make raised lines or bumps under the plating. They are referred to as Plating Blister when they are round and Linear Plating Blister when it forms an line.

 

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Basically, when the zinc planchet is coated with the copper layer, debris or other things on the planchet will cause the copper to NOT stick, leaving a bubble, spot or lines in the surface.

 

This is so clear! Thank you so much!

 

Thank you so much for this explanation! You both answered at the same time!

 

I've seen little bumps and think of them as "Pimples" because there are often more than one, or a few. The ones I've seen are usually very small and seem inconsequential. I've never saved cents with those "Pimples" because I've never seen anything so defined and obvious as this coin. Can I guess those little "Pimples" are in fact "Plating Blisters"?

Gosh, "Blisters" sound so much more painful than "Pimples". And since these things happened when the coin was "born" - you would think "Pimples" would be gentler sounding than "Blisters". It's fairly common for even infants have "Pimples". But babies almost never have "Blisters". And Blisters hurt a lot more too! Too bad nobody consulted me when they came up with the terminology for these oddities!

 

Yes, zinc pennies are prone to acne. It's terrible.

 

You don't see it too often but it does happen.

 

I think you just did explain it. That’s my guess

 

Maybe a straight line from the Memorial on a Lincoln. But if you put a loupe on it, it is definitely crooked.

 

OK ok, it is rare. Honestly though as a carpenter who inspects straight lines every day, I will say that any type of material I have ever worked with has never cracked or split in a perfect linear line. Just my two cents.
In my 30 years starting as a apprentice, it has never happened.