And Randy found the second one -- well done! The error is the name spelling of the man in the portrait. Hugh McCulloch was the Secretary of the Treasury at the time and his name ended up "McCullough" on the stamp. And since McCulloch was the chief administrator of the agency that oversaw the Bureau of Engraving & Printing, you'd think that would be a familiar name to the letter engravers.
But just as ironic is the BEP misspelling it's *own* name! If you look closely at the Bureau imprint you'll notice that the last word is "PRINIING". This error also made its way onto REA21 and REA29 of the 1871 series. Quite amazing that these typos slipped past considering the Bureau's high standard of quality control. But to be fair, the lettering is so small that no one else noticed until years later.
I encourage you to take a look at some of the other revenue proofs available for study on the NMAH website. There's some beautiful stuff on view, including a number of Persian Rugs.
http://americanhistory.si.edu/colle...:objectgroup