Double Spending and Confirmations Myths

[🏰 TradeFortress 🏰]

The network will become faster when there are more miners or hashpower
Myth.
The network regulates itself with the mining difficulty to try and get 1 block every 10 minutes, on average. An increase in hashpower will result in more blocks being mined until the difficulty retargets higher (every ~2000 blocks)

If I don't see a double spend transaction on the network, it is safe to assume that this transaction won't be doublespent
Myth.
A miner can include any transaction in their block, including something that has being never broadcast to the network. A blockchain fork is required if it conflicts with an existing transaction included in a block.

We would know if someone has more than 50% hashpower
Myth.
An attacker with more than 50% hashpower could be mining his own chain and not announcing it to the network (from the last checkpoint in bitcoind). As his chain will be longer, when he announces it will immediately propagate and override the current chain.

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[Stephen Gornick]

We would know if someone has more than 50% hashpower
Myth.
An attacker with more than 50% hashpower could be mining his own chain and not announcing it to the network (from the last checkpoint in bitcoind). As his chain will be longer, when he announces it will immediately propagate and override the current chain.

There are two approaches, too.  The March 11th hard fork was essentially a friendly 51% attack.  The "attack" (the switch from the longest chain to the other side of the fork) didn't occur until long after transactions had confirmed post-fork.
 - http://www.bitcoinmoney.com/post/47048259653

Then there is the stealth attack where the miner with 51% has been mining and not releasing blocks as they are mined.

Double spending
  - http://en.bitcoin.it/wiki/Double_spending