Accidental forks are created from bugs in software making two distinct forks with clients from one version only seeing one valid, while clients from the other version only see the other as valid (see BIP 0050).
If a pool has 51% of the hash rate (or more) it's not a problem for the network unless the pool owner (or a hacker with administrative privileges to that pool) decides he wants to start hiding his chain from the network.
PoW 51% + doublespend in a few words:
1) Everyone else has <50% of hash and is mining a chain.
2) Attacker mines a longer chain in secret, not reporting it to the network. Attacker can do this because he has more hash power.
3) Attacker spends on the <50% fork, waits for 6 (or whatever) confs to get it to spend at the exchange, then exchanges it for cash or whatever.
4) Attacker dumps his chain onto the network. Entire network invalidates the <50% chain, replacing it with the attacker's chain. Attacker's coins are now returned to him to respend at will. Not only this, but all the blocks mined in the <50% chain are now invalidated, so everyone not the attacker loses their blockrewards.
So, end result is that attacker gets 100% of the network coins from block reward and can double spend freely.
If a pool has 51% of the hash rate (or more) it's not a problem for the network unless the pool owner (or a hacker with administrative privileges to that pool) decides he wants to start hiding his chain from the network.
PoW 51% + doublespend in a few words:
1) Everyone else has <50% of hash and is mining a chain.
2) Attacker mines a longer chain in secret, not reporting it to the network. Attacker can do this because he has more hash power.
3) Attacker spends on the <50% fork, waits for 6 (or whatever) confs to get it to spend at the exchange, then exchanges it for cash or whatever.
4) Attacker dumps his chain onto the network. Entire network invalidates the <50% chain, replacing it with the attacker's chain. Attacker's coins are now returned to him to respend at will. Not only this, but all the blocks mined in the <50% chain are now invalidated, so everyone not the attacker loses their blockrewards.
So, end result is that attacker gets 100% of the network coins from block reward and can double spend freely.
How the attacker could have money to spend in step 3 if he s holding a secret chain? His coins wouldnt be valid until he dumps his chain?
He has to have the coins beforehand.
So if you want to 51%, first buy up; then mine your chain while selling your stock; then overwrite and again send the coins somewhere.
The coins earned by mining the blocks in your longer chain is no more than what you would get by mining on the normal chain anyway. The only profit is the double spend. Which makes the attack on FTC quite baffling as there was no significant double spend.
