What does a star after the serial number mean?

If the Bureau of Printing and Engraving make an error in the printing of a banknote, the note has to be replaced. Any replacement notes are marked with a small star at the end of the serial number.

Because these are reasonably rare, a perfect mint condition uncirculated 1999 $20 note would be worth about $60 instead of $30 for a standard note without the star.

A worn, creased note that has been used is only worth its face value of $20.