Consensus is achieved when all participants of the network agree on the order and content of the blocks in the blockchain.
Within a Blockchain framework, which is composed of a network of distributed computers, there is no one authority to provide oversight and ensure the correctness of entries.
There is therefore a need for consensus over the precise state of the network at any given time. Achieving this consensus is not a small task given all of the possible variations that might challenge the network
Deliberate attacks / Hacking / Sabotage
Malfunctions; Power cuts, equipment failure, natural disasters etc