Dark Digit in Serial NUmber

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I got a BTA $10 in my change at the cafeteria at the university today. The last digit (number 5 in this case) looks darker than any other digit, and than the other 5 in the first serial number. I would grade it a F. Haven't scanned it yet, looking to see if this is common before I bother.

Thanks in advance

 

 Yes this is very common. So common in fact that two other members asked the same thing in the last month.

That's what I thought. Thank you.

 

The following notes pulled from bundles all have heavy ink on the last digit (#8) at the left s/n.   

Seems like the problem only shows on the left side with number 8 as the last digit.  All others are normal.
{http://members.shaw.ca/ekong/kobecurrency/AOY%20-%20Dark%20%20last%20digit%20%238.jpg}

Funny thing is that most of the ink fills were on the right side not the left side in my bundle and the openings were completely filled in with ink.
Does anyone actually know how the counters work and are filled?
I tried looking at the BOC site, but not much info or pics on how notes are processed/printed.  Must be top secret stuff .

With these ones here, the problem  is only on the left s/n, and on every last digit with number 8. 

All the rest, including number 8 not in the last digit, are normal.

The following notes pulled from bundles all have heavy ink on the last digit (#8) at the left s/n.   

Seems like the problem only shows on the left side with number 8 as the last digit.  All others are normal.
{http://members.shaw.ca/ekong/kobecurrency/AOY%20-%20Dark%20%20last%20digit%20%238.jpg}

I got an AOH note a few days ago that had the same thing with the last digit (in my case being a 1) was very very dark. Indeed also on the left side!
This seems to be a common occurance in the journey series notes.