Why are my letters printing on the wrong side of the letterhead to the HP 506?

This will happen if your print setting are set on double-sided printing.  Uncheck the box that says “print on both sides” and it will work correctly.  To change the default setting so that it defaults to one-sided printing, go under Start -> Devices and Printers.  Right click on the 506 and select “Printing Preferences”.  Where it says “Print on Both Sides”, change it to “No.” This change will then take effect once you close out of applications and re-open them.