But there are ~1 billion microwaves in the world... I'm sure it has happened somewhere. As a designer of a billion-sold device, your job is to make sure that the expected number of people harmed by your device is substantially less than one, which gets really hard when all the risks are multiplied by 1e9.
Your job is to make sure the number of people harmed _while using the device as intended in a reasonable situation_ is as close to 0 as possible.
A domestic microwave is for use only on land, indoors, in a domestic kitchen, and in an unmodified form. In these conditions there is no conceivable way that salt water could saturate the main board, or bypass all the interlocks in another way.
Yes there are ways that all the safety systems can be bypassed, but not while a reasonable person is using the device as intended.