Uhh, they weren't specifically isolated events, just certain updates that had
unexpected features, which resulted in a lot of people discovering said unexpected features, for better or worse. And I'm having a hard time finding the exact updates that featured these oddballs, since they weren't exactly listed on the roadmap.
The "Gibbon Era" was when Toady was fleshing out jungle mobs a bit more, and did some research to make sure everything was nice and realistic. This was when he discovered just how
god damn many kinds of gibbon there are. In a heroic attempt to maintain his legendary standard of detail and realism, Tarn set about adding as many of them to the game as possible.
This resulted in a flooded animal pool for jungles, meaning that almost every single animal encountered in a semi-jungle environment was a gibbon in one way or another. And there were just so, so many gibbons...
The "Dwarven Weightloss Sauna" update was right around when flesh layers (skin, muscle, fat, hair) were first implemented. Temperatures were also being tweaked, and there were special provisions allowing creatures to treat air around lava as being much cooler than it actually would be, in order to allow for things like magma forges to exist and be used.
However,
water conducts heat very well, and what people found was that dwarves (and other creatures) that were standing in water or
that had a water covering from swimming or waterfalls were getting
much hotter than those dwarves who stayed dry.
The reason these saunas were of the "weightloss" variety was because although not directly harmful/fatal (for the most part), the temperature these wet dwarves reached was higher than the melting point of bodyfat. This resulted in the fat layers
inside living dwarves melting into a liquid state and then
falling out of their bodies onto the ground, leaving the skin and other layers intact.