[Scoundreller]

The usualy idea of the valves is for them to open before the overpressure exceeds their design capacity.

A blow-out disk isn't going to fail to operate. My pressure-cooker has one. It's just a piece of material that will catastrophically open the vessel to the atmosphere before massive overpressurization.

[Piskvorrr]

Well...that's a very unfortunate example. Pressure cookers' blowout disks can get clogged by whatever you're cooking (beans specifically) and the whole lid blows out (embedding itself in the ceiling and painting the kitchen). Scary stuff.

[Scoundreller]

I have a Presto pressure cooker. The blowout disk is a piece of rubber/plastic built into the cap. I have a hard time imagining it getting contaminated in a way that surpasses the strength of the stainless steel lips on the lid/pot.

Its design hasn't changed since 1977.

It's not a tube that a bean could get into. Maybe you're thinking of the vent pipe? When that gets clogged, the overpressure plug is what blows away to release all the pressure.

[Piskvorrr]

Well...I have seen the whole lid, firmly embedded in the ceiling. IDK what the brand was, but apparently the failsafe (a rubbery thing, off-center of the lid) failed to fail-safe in that case.

Looking into RAPEX, I see multiple recalls for this specific issue (pressure buildup leading to uncontrolled blowout); all for brand names that are unfamiliar to me.

[Scoundreller]

Of the 12 recalls I found on that database, none specifically mentioned failure of a blowout disc/plug1.

1 depended on the rubber gasket itself as the blowout valve. Several could be opened while under pressure, and some were deemed structurally deficient, which I take to mean that the vessel wasn't (consistently) built to withstand the design pressure (and a margin of safety).

[1] 1 did reach 290kpa (42psi), nearly triple the typical pressure cooker. Not sure what happened on that one.