One sided U.S. banknote

I realize this is a very vaugue question as I dont have the banknote in hand and no photos yet which I hope to get soon. I know nothing about banknote collecting but  was at a family function and a relative stated that he had a U.S. $20 bill which he inhereted from his father who used to work at a bank in the 1970's early 1980's although I dont recall exactly. One side is printed and the other side is not. I have no idea of which side is not printed or the exact date of the banknote. My understanding is that it is relatively pristine, well preserved, and has no folds, tears, or blemishes. As soon as I get photos I will post them here. Meantime, I am attempting to get an idea of  how unusual these errors occur while getting past quality control after printing and sent to banks and if these errors add real value to the banknote. Thanks in advance, your thoughts and insight are appreciated.

here is a excerpt from worthpoint.com.

Once the sheets of paper are prepared, the process of printing takes several steps. The first print covers the note's obverse side, and the second print covers the front of the note, including the borders and the portrait. The third step is the overprint, which applies the district and Treasury seals and the serial numbers.  At this point, ink errors are possible. For example, the ink doesn’t fill the plate, or it overfills, creating smears or blots. An impression cylinder might be damaged. It might fail to make a positive press or pull the paper into the intaglio portion or engraved portion of the plate. Offset print errors occur when the reverse image is shadowed on the facing side of the bill. More unusual are bills missing print, which can happen when any of the three steps are skipped. This mistake creates a blank back or plain back, a bill with no image, or one with only the treasury seals and the serial numbers in place. 

So it is possible. If you get it, or see it, first check is to see if the paper seems real and it wasn't some kid using a scanner.

Thanks. Still trying to get the photo of the banknote from family.