the straw that broke the camel's back = the last fatal error ?
It refers to the pre-industrial era when
draft animals (horses, oxen, camels, elephants, donkeys, llamas, etc.) were used for carrying heavy loads. Some of these animals were hitched to wagons, carts or carriages which carried the load (food, other commodities, or people), while others simply had the load strapped to their back.
If I'm not mistaken, elephants, llamas and camels cannot be used to pull wagons for various reasons. So if you use a camel for transporting a load, it will be strapped to the camel's back.
If your load is a bundle of straw to be used in construction or agriculture, you have to decide how much straw your camel can carry, since they differ in size and strength.
An unkind owner will try to put as much straw on the camel's back as the camel is capable of carrying. If he overestimates, the result will be that the camel lies down, or even falls over. The phrase "break the camel's back" is a
colorful exaggeration. I'm 100% certain that no camel ever had its back broken by having one more straw added to its load.
But I'm sure many camel owners were
bitten or
kicked for attempting to overload their animals. Camels are famous for not tolerating cruelty--and their definition of "cruelty" is not very generous.
Anyway, back on topic... "The straw that broke the camel's back" is a
metaphor. It means that someone overloaded a process, a machine, etc., and caused it to break down.