I'll start with a one of my own. Instead of "the straw that broke the camel's back," my world's version of the idiom is "the thorn that tipped Caranno's scales.

Caranno is one of the more important gods in my setting by nature of his being the patron father of one of the two most powerful empires on Akris. He's the most commonly worshipped god in the Philotite human empire, and he has considerable sway in other non-human nations thanks to the Watchers of Caranno (think a holy military that fights demons and monsters).

The Philotites are my stand-in for normal modern humans, by the way. There are a few other "human" races like the Sikvar and the Arthmorrans, each with their own quirks that make them not Homo sapiens. The Sikvar (Homo silva) are tuned to the forest they live in and the Arthmorrans (Homo diuturnus) are their own can of worms.

Back to the Philotites, Caranno is also the god of justice and mercy, and his symbol is a set of golden balance scales wreathed in silver thorns. He's not exactly a nice god ("granting Caranno's Mercy" is another idiom in my setting, and it's used instead of saying you've killed something.) He is an important one, though, since he unequivocally made the humans (each race in my setting was made by a specific god in his/her own image) and the Watchers of Caranno have a very justified existence.

The origin of my setting's idiom comes from what his symbol represents. The balance scales symbolize Caranno's dual nature of being the god of mercy and the god of justice. The thorns symbolize sins (repented and unrepented) committed by the people he judges. Get too many metaphorical thorns on the wrong side of the scales and Caranno will decide you deserve justice instead of mercy.

"The thorn that tipped Caranno's scales" started off as a reference to the concept that a person can commit one unrepented sin too many and rack up enough thorns to tip the scales against them, making Caranno punish them instead of having mercy on them.

Over the centuries it became a catchall that functions the same way as our world's "straw that broke the camel's back" and it now means the same thing.