Thermos jars have interior threads for an inner plug and an exterior thread(singular) for the cosmetic cap that doubles as a drinking cup. Maybe there's a condition where the external cup would be holding the inner plug from releasing, and then upon removing the cap, the entire lip starts tapering outward and lets go of the plug without gradually relieving pressure.

(to be fair, I don't think an externally threaded cap will solve this problem. If the thread held to a higher pressure, then the core part of the plug will eventually blow out even more energetically. An overpressure vent port is still necessary)

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kevin_thibedeau an hour ago | parent [-]

All they need is a section of threading that gets looser to release pressure while it is still retained in the top.

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malfist an hour ago | parent [-]

Thats all? What happens if the jar is turned upside down in this case? What if something is set on the lid? What if the jar is with someone heading to a camp site in a 4 wheeler and it is vibrated a lot?

	▲	kevin_thibedeau 25 minutes ago \| parent [-]
		You have a section of threads that seal or have a gasket. This is simpler than a relief valve that can leak or collect bacteria.