Star Notes.

Ever wonder what that star next to the serial number found in some of your dollar bill? Here’s the answer:

When an imperfect note is detected during the manufacturing process after the serial number has been overprinted, it must be replaced with a new note. A "star" note is used to replace the imperfect note. Reusing that exact serial number to replace the imperfect note is costly and time consuming. The "star" note has its own special serial number followed by a star in place of a suffix letter.

The serial number of the imperfect note that was removed is not used again in the same numbering sequence.

Link - Thanks James Pelchar!