Aaron v. Aron
There's actually a whole footnote on this in Wikipedia:
Note a: Presley's genuine birth certificate reads "Elvis Aaron Presley" (as written by a doctor). There is also a souvenir birth certificate that reads "Elvis Aron Presley." When Presley did sign his middle name, he used Aron. It reads 'Aron' on his marriage certificate and on his army duffel bag. Aron was apparently the spelling the Presleys used to make it similar to the middle name of Elvis' stillborn twin, Jesse Garon. Elvis later sought to change the name's spelling to the traditional and biblical Aaron. In the process he learned that "official state records had always listed it as Aaron. Therefore, he always was, officially, Elvis Aaron Presley." Knowing Presley's plans for his middle name, Aaron is the spelling his father chose for Elvis' tombstone, and it is the spelling his estate has designated as the official spelling whenever the middle name is used today. His death certificate says "Elvis Aron Presley." This quirk has helped inflame the "Elvis is not dead" conspiracy theories.