Hey guys,
I thought I would put up this warning, as I nearly lost a Sim to this phenomenon.
To clarify, if your Sim drinks a lot of coffee within a few hours when it's hot outside - even if the thermostat is set to cooling and it's nighttime, and they are wearing a hot weather outfit - they can become dangerously overheated (a -99 red moodlet) and die. Taking a shower or bathing doesn't help, but drinking a glass of water cools them down immediately (to a -1 red moodlet).
So, there you have it.
Update:
It appears this phenomenon has been taken care of with the latest patch (June 18).