Error notes are notes that are misprinted at the BEP. When ,these sheets ,are detected ,they are pulled and destroyed ,and then they are replaced by perprinted star notes. However, some of these error notes escape detection and are released into ,general circulation. There are various types of error notes, some more dramatic than others. , Collectors are facinated by these types of notes and a whole new collecting market ,was born. Some of the types are: Blank backs, missing first , second or third print, foldovers, miscuts or mis-aligned notes, gutter-folds, inverts, double denominations and a few others. The more the dramatic the error, the more valuable they are. There are several books on these types of notes by authors Fred Bart and Steve Sulliavn. You can buy these books at your local book store or on-line.