slightly out of line last number of serial, both sides

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I just got a $20 Macklem-Carney BSC 1610539 with both 9's lower than all the other numbers.  It is only  fractionally lower than the other numbers, but still quite obvious- about 0.5 mm lower. Would any of the later notes in this series show a more major drop? How quickly would a small error like this be picked up?

Just the BABN $20 notes seemed to have had this slight digit drop (mostly 9's but I've seen some lowered 6 digits too).  These were reported back in 2013/14 not long after they appeared but most collectors felt that they were within QC tolerances & not significant enough to be considered an "error."   I found a few but put them back.

I think most error collectors prefer a SN digit to be either stuck so that the drop is completely below the other digits or the digit to be missing (a more dramatic error).