Turns out people leave perishable and fermenting foods in the thermos, and after a while when opened, the pressure lifts the lid of the thermos at quasi-unimaginable speeds, striking the curiously unsuspecting humans straight in the face, with some instances causing permanent damage to vision.
Coffee is hot, but a pressure release system is cool too..
Scroll down in that article to the section with photographs of "recalled" and "not recalled" lids side by side.
That by far is not enough to forcefully yeet out the cap, probably not even if you take it to an Antarctic research base in -40 °C outdoor weather.
People forgetting about content that ferments however? Kaboom.
It can be seals that buckle before the lid. It can be a tiny hole with a soft rubber stopper. It can be one of many things that cost a couple of cents extra and a bit more engineering and testing effort.
The cheapest disposable coffee cups I've used have a tiny hole for the express purpose of not pressurizing and spilling hot liquid everywhere.
There's a lot of conversation in the comments about "was there was an expectation that the pressure release valve would be there" There absolutely is a safety expectation that a sealed container of hot food is designed with a pressure release system.
BTW the fermented food thing is a misdirection. The pressure release system should have released pressure way before it even reached ballistic territory.
That's not very empathic of fellow people who might just have mistakenly forgotten food in the container.
It's not like they were intentionally using this for brewing some illegal substance or misusing it in a way specifically forbidden in the manual or obviously unsafe (e.g. removing a magnetron and all of its protections from a microwave to make pretty wood carvings).
Can you also guarantee with absolute certainty that you'll never forget them anywhere another person who's unaware of the contents or the danger could find them?
The malfunction is due to multiple models of containers missing a pressure relief function in the center of the stopper.
I didn't consider it a defect, though. I can hardly imagine cheap products adopting this solution.
But then I read the words "The malfunction is due to multiple models of containers missing a pressure relief function in the center of the stopper."
How non-existent does your quality assurance have to be in order to miss such a critical, obvious and easy to identify flaw ?
Looking at the published photographs, you don't even need training to identify that manufacturing defect. A five year old could spot the difference between "lid has a hole" and "lid does not have a hole".
Me standing there, kombucha and peach slices pulverized against the cap, kombucha leaving a large mark on the ceiling it all happened so fast, glad I was wearing my glasses.
No injuries except my pride, but it did take some hours to clean up.
I wouldn't do it in a thermos, but I guess those are mostly accidents?
Tasted okay though.
My mate brewed his bramble beer by putting his brambles through a juicer and only adding the juice and pulp, because he's cleverer than me and has a certain amount of foresight.
Looking at the version that's not recalled, I would guess it costs more to make.
My partner has one of these so just checked it. It does have the pressure relief feature, but it turns out it's also missing all the seals anyway so never would have been a problem!