Anyone else every discover a doctor either missed something during an exam, or that a doctor found something inside you you had not clue was there.
This guy had a four-inch knife in his head. Evidently he was mugged and stabbed, but the doctors didn't find the knife blade, even though the guy was bleeding out of his ears.
http://www.latimes.com/health/boostershots/la-heb-knife-skull-20110217,0,655449.story
The above also mentions a guy who shot a nail through his head and didn't know it until he got a tooth ache.