Is there an equivalent phrase in Spanish for "the straw that broke the camel's back"?
The phrase usually refers to to the final thing that is added to a bunch of things to cause a large reaction and can be applied to nonphysical things like stress.
The most similar I can think of in Spanish would be
La gota que colma el vaso
The verb colmar (overfill) means
Llenar una medida, un cajón, un cesto, etc., de modo que lo que se echa en ellos exceda su capacidad y levante más que los bordes.
So the overall expression means "it was just a tiny drop of water, but the glass was already completely full to the brim and overflowed". This is something you'll use to convey "OK, I have been really patient, and this is the last thing I'm going to put up with!"