By 'misbehaves,' if somehow this miner controls 51% hash power of the network, and then he tries to attack blockchain network, adding invalid blocks or creating a fork in blockchain then trying to add the block on top of his block – Prajit Verma – 2019-02-15T10:32:38.090

A miner with 51% of the hashpower cannot add invalid blocks, but they can fork all other miner's block off the network, except for their own. In any case, the network's nodes will continue to follow the longest chain of valid blocks. There is no way of knowing who has submitted the blocks, so if they are valid, the nodes will accept them. – chytrik – 2019-02-15T10:34:44.093

What will happen to transactions present in the fork in which all other miner's block are present which he(mischievous miner) set off the network. Will those transactions become useless? – Prajit Verma – 2019-02-15T11:06:32.003

for eg. merchant who waited for 6 confirmation to verify payment, then ship the product bought. But if those transactions became useless or in some sense were thrown out of the public ledger, then merchants would have to bear loss! – Prajit Verma – 2019-02-15T11:10:16.887

@PrajitVerma Yes, if there is a fork (reorganization) then the transactions that were confirmed pre-fork, may no longer be confirmed post-fork. This is why it is often recommended to wait 6 confirmations: the chance of a long reorg is low, and it is very expensive to attempt such an attack. See: https://bitcoin.stackexchange.com/questions/658/what-can-an-attacker-with-51-of-hash-power-do also relevant: https://bitcoin.stackexchange.com/questions/3343/what-is-the-longest-blockchain-fork-that-has-been-orphaned-to-date