La goccia che fa traboccare il vaso (literally “the drop that makes the vase overflow”) refers to the latest occurrence in a series of cumulative small, unpleasant events that unexpectedly triggers a significant and sudden reaction.
La goccia che fa traboccare il vaso
It is identical in meaning to the English idiom “the straw that broke the camel’s back“.
C’erano molti problemi fra di loro, ma la goccia che ha fatto traboccare il vaso è stata la sua infedeltà.
There were many problems between them, but the straw that broke the camel’s back was his infidelity.
Another way of saying the same thing in more literal Italian is far precipitare la situazione (to worsen the situation).
According to Virgilio Sapere, the proverb can be found within the work of Italo Svevo, “Senilità.”