Error Currency is widely collected.  Error currency is not only intriguing, but it is even impressive to the noncollector.  Legitimate error currency are notes that leave The Bureau of Engraving and Printing with some kind of flaw.  Often times these flaws are a result of poor printings, too many printings, or poor cuts.  Error currency is not often encountered so it tends to command a significate premium.  Due to this premium more and more people are making their own error notes.  The following are not real errors and should always be avoided:

The Missing Black Federal Reserve Bank Seal:
This is a clever trick that is accomplished by a solvent and a replicated stamp.  The con-artist will chemically remove the black federal reserve bank stamp.  A replicated seal will then be stamped somewhere else on the note where it obviously doesn't belong.  This leads the novice to believe this is a true BEP (Bureau of Engraving & Printing) error.  The note will often be circulated so it is harder to notice where the seal was.

The Miscut Note:
This one has been around for a while and is so easy to do that you see them everywhere.  The con-artist simply buys a sheet of currency from the BEP for about 40% over the face value of the sheet.  He then cuts the sheet so that it appears that there was a tragic cutting error at the BEP.  This one is very easy to spot because notes that were once in sheet form have a serial number over 99,000,000. 

Ink Smears:
These are hard to spot, but one type of error is an ink application error.  The true error will have too much ink in one area that has been smeared across the note.  These can be forged in any number of ways.  Some of these errors are legitimate, but some are being replicated, so be wary.

Double Printings:
Double printings involve the serial number or seal of the note having a shadow where the exact thing was also printed.  Many of these will be legitimate.  However, if only one part of the note is doubled, that is likely counterfeit.  Most double printings will have more than one area affected.

Obstruction Error:
These are some of the most impressive of all errors.  The only problem is that they can be replicated using chemical solvents.  The true obstruction error will have no sign what-so-ever of printing once being there.  If there is even a faint remnant of printing or color over the affected portion then it is a fake error.