$1 Bill Error?

I am primarily a coin collector, but a friend of mine asked me to help him find out about a dollar bill he recently received.
The bill is a 1971 One Dollar issue and the serial number in the upper right has the last digit dropped about half way down the rest of the digits in the number. In other words, the top of the last number (a 3) is about even with the center of the rest of the numbers in the row. Only the right serial number is affected the rest of the bill is what you would normally expect to see.
We are wondering if this is a common occurrence or is this a rarity?
Any information is greatly appreciated.