The presence of the naked ladies made them a bit controversial, and they were allegedly discontinued because the public felt that their money shouldn't look like racy French playing cards. Error Note A note with a printing error, ink smudge, or some other major imperfection. While the Bureau of Engraving and Printing inspects its output for such errors, mistakes often slip through. After all, they do print a lot of money. Error notes can be quite valuable, depending on the nature and rarity of the error.
Some collectors specialize in error notes.
Apparently, people found these so interesting that few stayed in circulation for very long. Not surprisingly, most of the other runs of experimental notes aren't so obvious. These are identifiable by serial number. Note A type of note issued by one of the Federal Reserve Banks. See the page dedicated to this type of note.