Do you know this latin proverb "A chain is only as strong as its weakest link" i.e if a link - not the token chainlink but in our case a block - is broken, the entire chain breaks ? 

As many users I am not so familiar with the Blockchain technology. I know that if someone (Satoshi Nakamoto for instance) sell a huge quantity of Bitcoin, a crack could happen.

So my question is more about the Blockchain technology   :

WHAT COULD BE THE WEAKEST BLOCK IN THE BLOCKCHAIN ? 

(or the question is not this one about some failure in the blockchain !)