https://en.bitcoin.it/wiki/Weaknesses#Attacker_has_a_lot_of_...
https://bitcoin.stackexchange.com/questions/658/what-can-an-...
"An attacker that controls more than 50% of the network's computing power can, for the time that he is in control, exclude and modify the ordering of transactions. This allows him to:
* Reverse transactions that he sends while he's in control Prevent some or all transactions from gaining any confirmations
* Prevent some or all other generators from getting any generations
The attacker can't:
* Reverse other people's transactions
* Prevent transactions from being sent at all (they'll show as 0/unconfirmed)
* Change the number of coins generated per block
* Create coins out of thin air
* Send coins that never belonged to him"
Doing a reorg of the blockchain can cause all sorts of problems, and reverse all sorts of things, depending on what those transactions were, and how they are re-ordered.
https://academy.binance.com/security/what-is-a-51-percent-at...