The idiom the straw that broke the camel’s back is from an Arabic proverb about how a camel is loaded beyond its capacity to move or stand. This is a reference to any process by which cataclysmic failure (a broken back) is achieved by a seemingly inconsequential addition (a single straw). This also gives rise to the phrase “the last/final straw”, used when something is deemed to be the last in a line of unacceptable occurrences. Variations of this idiom include “the straw that broke the donkey’s back”, “melon that broke the monkey’s back” and “feather that broke the camel’s back”. (source: wikipedia)
In Spanish we say: “la gota que colmó el vaso”.