Initially I thought you meant cards that should have an SN on here like the early-mid 90's NFL Collecror's Choice versions- ironically I recently (as in last week) did a "detailed add" for several of them and put the printed SN on my cards. Example:
https://www.tcdb.com/ViewCollection.cfm/sid/33888?CollectionID=1
Then I thought you meant something similar to a question I asked regarding sets like Topps Opening Day with their foil parallels (mainly the blue) that claim a certain number but there is never anything printed. That one taught me a lesson that SN and Print Run are two different things (not new info exactly) but people often incorreclty use them interchangeably thus the confusion. Example:
https://www.tcdb.com/Checklist.cfm/sid/199872/2019-Topps-Opening-Day---Opening-Day-Edition
Lastly, the replies explained a third option of either printing /manufacturing errors which I also have but never asked about. For example:
https://www.tcdb.com/ViewCard.cfm/sid/33850/cid/3457124/1995-Classic-NFL-Rookies---Die-Cuts-Silver-Signatures-1-Ki-Jana-Carter
I have 4 of these and even on the back of someone else's image and in the Release Notes it says SN 1750 but not one of my cards has a number nor signature.
Just thought I'd share how one question can evoke multiple ideas and from this I have a feeling not one of my ideas is what you meant so hopefully I will learn something new.
Cheers! Becky Elizabeth