I work for the American program.
At South Pole there is a plaque with the names of everyone who has wintered over with the year they did it. Pole was inaccessible in winter until KBA did it in 2016, but I dont think it will ever happen again. You can read about the rescue, it was incredible.
Either way, these fifty or so winterovers at Pole now are probably the most isolated human beings in existence, even moreso than the ISS. So its a real achievement to handle the rigors of it from February to November.
OK NOW FOR THE SPOOKY PART
There are four names scratched out of the plaque for a few years ago (I know the year, I wont say it). I have a first-hand account that from May onwards there nobody was allowed to travel around the station alone. People had to go to the bathroom, eat, shower in groups. All winterover science was cancelled.
The people who know what happened wont say, and its even mostly absent from the usually-voracious McMurdo rumor-mill.
The running theory/speculation is that the four names that are scratched out rebelled against station management and started trying to injure or even kill people. But that doesn't make a ton of sense since they weren't flown out at the earliest possible time (October flights are doable), they waited until the LC-130's came down. But also worth noting that the first Air National Guard flight to the Pole only fueled in McMurdo. They didn't stop, they didn't do a crew-change or let anything off. Nobody knows what was on that plane, the people who winterovered that year will not talk about it. It would be considered extremely taboo to bring it up.
Either way, something happened. There's not even any real telling if everyone from that year survived and came back. There is nothing about this on the internet anywhere, I am actually a little leery about bringing it up. But fact is I know nothing and this story is not uncommon knowledge around the USAP and its definitely been told outside the program.