As the question asks, I imagine that in the confusion of war it must have happened sometime somewhere that someone has been awarded a medal posthumously in the believe that they had been killed but they turn up later still alive.
But has this actually ever happened?
Not quite a medal, but from this week’s LRB review by Ferdinand Mount of the new de Gaulle biography:
In fact, he’d been captured and spent the rest of the war as a POW (with various failed escape attempts).
Thank you, well at least we know it has happened 
Didnt Joshua Chamberlain get his Medal of Honor because they thought he was going to did … then he lived.
That would actually be pretty interesting to know if this ever happened. I googled it but couldn’t find anything.
If I remember correctly Gregory “Pappy” Boynton was awarded the Medal of Honor after he was shot down and presumed dead. He was captured by the Japanese.
I would imagine it depends on the medal, it wouldn’t surprise me if a lot of purple hearts went out to people who turned up later, but that MOHs are a bit more researched.
[referring to the Medal of Honor].
The phrasing in wikipedia makes it sound like he was known to be alive at the time, but one of the citations in the wiki article is " “Boyington, Marine ace, reported alive in Japan”. Ellensburg Daily Record. Associated Press. August 29, 1945. p. 1." , which kind of implies that it wasn’t until August 1945 that he was known to be alive. Another source “Hampson, Fred (May 23, 1944). “Boyington still alive, rumor over Pacific”. Pittsburgh Post-Gazette. Associated Press. p. 5.” says that it’s a ‘rumour’ he’s alive, from May 1944.
Which makes me think it’s more likely than not the Medal was awarded to someone presumed dead at the time (with the usual wartime uncertainly around ‘presumed dead’).
My son & I went to a book sighing he was having in Tulsa, OK in the mid 70’s and as we were early we helped him get set up. During our visiting he told us that his MOH was political and he had done nothing to earn it. He seemed quite bitter that the MOH was used like that and that it was him that it happened to.
My son was quite the fan of the TV show “Ba Ba Black Sheep” and he got to spend an hour or so with “Pappy” one on one. Signed book and all. A most interesting man and we got quite the inside look at what that part of the War was like from someone who was there and such a hero to my son.
He also had a great uncle that was a naval pilot who was killed at Pearl Harbor so has heard many things of the War in the Pacific.
Louie Zamperini, a former Olympic track star (read or see Unbroken) was believed to have been killed in a B-24 crash, until he emerged from the same prison camp as Boyington. After the war, he had the pleasure of firing the starting gun at the Louie Zamperini Memorial Mile in New York.
Has the opposite ever happened, where an award is given and the awardee doesn’t show up to receive it because of a previously unknown engagement with the Grim Reaper?
Kirby Grant, who played Sky King on TV, died in a car crash while on his way to Cape Canaveral to get an award from the astronaut corps for inspiring so many of them to take up flying. That was to be followed by watching the (last successful) launch of the shuttle Challenger.
I’m think more like the award being assigned with the award givers not knowing that the awardee is already dead.
I remember reading about an Italian sports stadium (Olymics?) to be named after a late Italian Olympian, but just before the ceremony they found out that he was alive. A quick Google search didn’t turn anything up.
The 2011 Nobel Prize for Medicine was partly awarded to Ralph Steinman with the committee not realising that he’d actually died a few days before the announcement. Deeming that they’d acted in good faith, the the award was allowed to stand.
(The rules are such that they can’t award it if they know the recipient to be announced is dead. And they do obviously run checks to prevent this happening, obviously hampered by the requirement that everything is supposed to be secret up till the announcement.)
Here’s one that is outside of the US. I don’t know the website so I can’t swear to the accuracy.
I guarantee that a bunch of Purple Hearts were awarded that way. Zamperini was not the only one presumed dead but was in a Japanese POW camp. The other Niland brother that lived was in the same camp. Anyone who died due to combat is awarded the Purple Heart.
Henry Erwin should have died. He should have died before he was awarded the Medal of Honor. He should have died right after. Curtis LeMay probably broke every protocol to get him the MoH before he died. They broke into a display case containing a MoH in Hawaii and flew it out to Guam before he died. He didn’t die until 2002. Not exactly what you are looking for but an amazing story.
No he got his promotion to BG because they thought he was dying his MOH came about 30 years after the war.