The straw does not have to be any special straw.
In the fable the man piles straw on his camel’s back until the camel is at the limit of what he can carry. The man wants to bring just a little more straw on the journey, to make it worth his while. Seeing the camel straining, he decides to add just one more straw. Camel’s back breaks.
This is how accretion, the adding on aided by gravity, can bring down many things. Resentments, for example, grow by accretion if we do not resolve them. I am strong, we may reason, I can look past this insult, this betrayal of trust, this small injury. We carry this one, and the next, and believe we can be philosophical about it. Most of the time we can be, but we must continue to carry whatever we do not resolve, it has a weight and causes a certain drag and friction.
A trauma of some kind comes up, the other party, arguably intending no great harm, does one more thing that weighs in on top of the pile of grievances we already bear on our backs. Boom! Done, broken.
When times are relatively good we can carry more without breaking. At the breaking point, the final straw can be relatively light, it will tip the scales and the thing will break, whatever it is. Forgiveness has its place, and it is a wonderful and essential place in a good life, but only if the thing forgiven is not endlessly repeated.
Otherwise the thing forgiven, papered over, minimized, agreed to disagree about, ignored, lost but not forgotten, is carried on an already burdened back. The straw is waiting to fall on to the pile, as it is always poised to do.