But for a long string of unexpected consequences I don't know of a proverb. There's a poem I learnt at school called 'For want of a nail', but maybe I have an inflated opinion of its currency! (
For want of a nail rhyme) . There's also the modern weather forecaster's reference to a butterfly flapping its wings the other side of the world (with a string of consequences ending in a change in the local weather).
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