How do "over heating" failures occur?

[DLLNP]

Hello!

We have all seen (or maybe experienced?) the horror stories of a thermostat failing, resulting in an animal being burned or killed.

My question is: How does that happen?

I think (as far as I know, correct me if I am wrong) most heat mats and/or heat tape if left unregulated are not normally capable of heating up enough to seriously hurt or kill your animal when functioning properly.

Let me be clear about what I mean... an unregulated heat mat normally won't reach temperatures above 120°F or so, of course this is way above what your husbandry requires but it's not going to melt plastic, burn scales or start a fire. (again... I am not at all suggesting not to use a thermostat, and I know 120°F is not a safe temperature for most animals but it is far from a temperature that will cause immediate serious damage which I imagine must be in the +150°F?)

So when a thermostat fails... I don't understand how the extreme over heating occurs (burns, death)?

OR is it that the heat mat fails, burns out the thermostat probe and then the temperatures can skyrocket? In other words, is over heating more of a faulty heat mat issue rather than a faulty thermostat?

I am curious to know you experience and thoughts?