There was an engineering design fault in the Apollo 13 oxygen tank: the switch was rated for the original voltage/current, and was not replaced when the design voltage/current changed.
This is a beginners mistake: a switch is just a switch, right? It doesn't need an AC/DC rating and a voltage rating and a current rating?
To be fair, the engineers involved probably weren't beginners, it was probably a project management failure, but clearly a lot of the engineers involved at the project management level were mechanical engineers, not electronic (as demonstrated by the discussion about how many 'Amps' they had left after failure).
The switch didn't open under load, because the switch contacts were under-rated. The heater didn't turn off, the shielding overheated, and later failed, exposing the heater wiring to the oxygen.
Here in Australia, something similar happened with a popular portable electric heater, leading to several fires and at least one death. Again, because the switch contacts were under-rated, the heater element did not turn off, the shielding failed, and fire was the result.