1999 Penny Error??

I was looking through my 99 pennies and I noticed this faint smudge underneath the date. It looks to be part of the coin and not just dirt. Is this some kind of error with the mint mark or not? I'm new to this so I am not sure. I have added pictures of it compared with a 99D penny.

 

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My guess would be a weak spot in the copper coating and the beginning of zinc rot. Welcome to CT

 

That sounds about right, I didnt think that it was a error but wanted to be sure. Thanks!

 

Hang on for awhile, I'm sure others will chime in.

 

if you look further up the cent too, it looks like you'll see a much larger zinc rot spot just in front of Abe's face.