Here you see very rare serial number errors.  Perhaps used for setup sheets?  Look closely at the $5 notes, there are really three prefixes.

These errors have both printing errors and serial number errors. More dramatic as a printing error, two very nice notes. 

Here the serial numbers were mirrored (transferred) to the front of the note.  Caused by the transfer of ink from the serial number on the back of the neighbouring note.

These notes have serial numbers which are entirely out of place or one side which is out of alignment with the other.  The $1 is part of a multi error note.

This note is one of the few mis-matched prefix I have ever seen.  Others are out there which are legitimate, however one must be careful, for some can be produced by removing part of a letter to make the error.

These notes show no serial numbers at all.  The $1 and $100 are legitimate, for I have examined them in person.  The $2 note looks to be legitimate, but I have note had the chance to see it in hand.  With this type of error, one has to be extremely careful for serial numbers can be chemically removed.  Usually fakes like these are lower grades and the colour seems to be bleached.

This error is a simple "over inked" serial number.  Not a major error but not seen too often.  Have a look at the error closer by clicking on the image and you can see the transfer of the extra in on the back of the note as well.

These notes show partial digits or digits missing altogether.