The Kennedy Half Dollar has been in the news recently due to the sale of 2014 50th anniversary commemorative coins made by the US mint. The orginal coin made an appearance on televison's "Twilight Zone" show in 1986 in "Profile in Silver", an unusual time-travel episode about a professor who turns up in 1963 with a Kennedy Half Dollar. The professor causes all kinds of problems when he saves John F. Kennedy's life in Dallas. Kennedy Half Dollar from the show Secret Service men in 1963 discuss whether the coin is a counterfeit or illegal, and who would have made it. One agent says that it is against US policy to put a living president on a coin. I posted a few pictures from the episode on my "Coins on Television" website.
One of the best political episodes of the New Twilight Zone. The numismatic twist just adds to the plot. Not only was President Kennedy saved, but the Soviet Premier Nikita Khrushchev was the target of an assassin. Everything that happened in the real world was flipped. The bust on the obverse of the coin oddly looks more like the man who later become the political leader of Romania in 1965, Nicolae Ceausescu.
By Back to the Future mechanics, you'd think as soon as JFK's assassination is thwarted, the JFK half would turn into a Franklin half (since that series would definitely have run at least another year, had JFK not died and subsequently have been commemorated on the coin). I guess in this story artifacts of an alternate future persist. Not strictly speaking true; it's against common convention to put a living president on a coin, but it has been done once: Calvin Coolidge was put on the 1926 commemorative (150th anniversary of US's independence) while still alive. Among non-presidents, Thomas Kilby and Eunice Schriver (JFK's sister) were both put on coins while still alive (although in 1963 they wouldn't know about Eunice's coin, not minted until 1995). In 1963 such a coin would not technically be "counterfeit" since there's no real JFK half to counterfeit. It may be illegal to produce though since it's an unauthorized representation of US money (I'm assuming they won't accept the explanation that it was legally minted in the future by the US Mint lol...). Assuming they don't believe it was minted in the future, they'd probably conclude it to be a fantasy and/or propaganda piece. I'm not sure whether or not it would be considered "illegal" under 1963 law. There's tons of time travel/alternate history stories about the JFK assassination (so far my favorite is Stephen King's "11/22/63"), but you don't often see it represented in a coin. I wonder if you could convince Daniel Carr to make a fantasy piece of a 1964 Franklin half dollar for all the alternate history buffs out there. Would probably sell well; heck, I'd buy one.