The BEP (Bureau of Engraving & Printing) produces a fair share of errors. There are overprints, offsets, faulty alignments, missing printings, mismatched serial numbers, inverts and dual denominations just to mention a few.
Billions of dollars are printed by the BEP each year and you can expect errors to happen. Some of these errors, especially dramatic ones command high prices. Conversely, there are a multitude of more common errors that are quite affordable for collectors. Since serious collectors like their error notes to be crisp uncirculated and are willing to pay the price, there are, nevertheless, still great buys remaining on circulated errors. I strongly recommend the reference book “United States Paper Money Errors” authored by Dr. Frederick I. Bart, available at Krause Books. Collecting errors is an interesting and rewarding hobby offering unique and one of a kind items for your collection.