In the ultimate example of "fake it til you make it," Ferdinand Demara boarded the HMCS Cayuga, a Canadian Navy destroyer during the Korean War. He was impersonating a doctor, which was fine until the ship started taking on more serious casualties and Demara was left as the ship's only "surgeon".
This is the point where most people would throw up their hands and announce the game was up, but Demara wasn't ultimately labeled "the Great Imposter" for nothing. He had a photographic memory and a very high IQ.
So the new doctor went into his quarters for a few minutes with a medical textbook, came back out and then operated the 16 badly injured troops — including one who required major chest surgery — and saved them all.
There is no word on which textbook you can read to learn how to perform surgery in a few minutes, but whichever one it is, it's totally worth the money. There is also no mention of how Demara managed to board the vessel and how no one recognized there was a new crewman aboard with no papers.
Demara's identity was somehow discovered after this incident and he could no longer live under different identities (he was even featured in Time Magazine). He previously worked as civil engineer, a zoology graduate, a doctor of applied psychology, a monk (on two separate occasions), an assistant warden at a Texas prison, philosophy dean at a Pennsylvania college, a hospital orderly, a lawyer, cancer researcher, and a teacher.
There was even a movie made about his life starring Tony Curtis. After that level of recognition, Demara could no longer blend in and integrate himself as he once did.
An interesting note, Demara never sought financial gain, just the experience of the job. He died in 1982.