In a degenerate case with enough power a single entity could mine several blocks faster than the main chain can mine one. In that environment they would be able

In a degenerate case with enough power a single entity could mine several blocks faster than the main chain can mine one. In that environment they would be able to reverse any given transaction essentially by making a transaction waiting for that to get several confirmations (blocks created after that transaction is included) and waiting for whatever real world purchase went through then releasing the other blocks they had mined without that transaction included. The client is programmed to chose the longest chain available so the old public chain is abandoned in favor of the attackers new chain which doesn't include the initial spend. Now according to the blockchain Alice never sent Bob the BTC so the money is back in Alice's address and Bob is out whatever real world or off chain changes were made. There's two ways to do this either tactically to reverse your own transactions or randomly to essentially screw over everyone and sow FUD in the reliability of BTC transactions. One good note is as far as I know the Lightning network shouldn't really be vulnerable to this (except maybe for channel opening?) because either party can submit the most recent transaction and have it reflect on the BTC chain.