Lady Liberty seen in the stamp is actually based on a photo of the Las Vegas replica at the New York-New York casino.
Linn's, a stamp magazine, first noticed the discrepancies - her hair is different, and her facial features are more sharply defined. The magazine also said the stamp shows a rectangular patch on the crown of the statue.
Three billion of the stamps have been printed.
The postal service says it likes the design and has no plans to pull it out of circulation.
The actual Statue of Liberty has appeared on more than 20 stamps previously, according to Postal Service spokesman Roy Betts.
The Associated Press contributed to this story.