Serial number question

Is it common for the last digit in the serial number to be slightly lower than the other numbers? I've never paid particularly close attention to this, but I just found three while looking through $150 of ones. That seems like a high percentage to me, so I'm assuming it is fairly common, but wanted to be sure.

 

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That's called a turned digit error. Depending on the severity, it can be valuable.

 

Thanks Funkee, I'll try to post some pics later when I get home. They don't appear to be to severe and the bills aren't in the best of condition either, but it's always good to double check.

 

Here are the pics. Glad I learned something new today.

 

The third pic is not coming up for some reason, but it is very similar to the first.