This image of Air Force One depicts the aircraft's tail number as "N166ER", a combination of letters and numbers that can be read as a well-known racist epithet. Given that the epithet is commonly directed against African-Americans, using such a tail number on African-American President Barack Obama's official aircraft would of course be a very unfortunate and embarrassing blunder.
Not surprisingly however, the supposed tail number is the result of a malicious prankster's work in an image manipulation application such as Photoshop. In fact, the tail numbers on Air Force One jets bear no resemblance to the one shown in the above image. There are currently two Air Force One jets in use, one with the tail number 28000 and the other with tail number 29000.
Tail numbers assigned to civilian planes do start with the letter "N" and are commonly referred to as "N-Numbers". However, military aircraft do not use the N-Number system and have tail numbers that do not start with "N". Information on the White House website notes:
No matter where in the world the President travels, if he flies in an Air Force jet, the plane is called Air Force One. Technically, Air Force One is the call sign of any Air Force aircraft carrying the President. In practice, however, Air Force One is used to refer to one of two highly customized Boeing 747-200B series aircraft, which carry the tail codes 28000 and 29000. The Air Force designation for the aircraft is VC-25A.
While the offending tail code does not belong to any Air Force One aircraft, it is actually a real N-Number. According to Federal Aviation Administration (FAA) records, the N-Number "N166ER" is currently assigned to a T-28C aircraft owned by a company in Houston, Texas.