Value overflow incident
On August 15 2010, it was discovered that block 74638 contained a transaction that created 184,467,440,737.09551616 bitcoins for three different addresses. Two addresses received 92.2 billion bitcoins each, and whoever solved the block got an extra 0.01 BTC that did not exist prior to the transaction. This was possible because the code used for checking transactions before including them in a block didn't account for the case of outputs so large that they overflowed when summed.
A new version of the client was published within five hours of the discovery that contained a soft forking change to the consensus rules that rejected output value overflow transactions (as well as any transaction that paid more than 21 million bitcoins in an output for any reason). The block chain was forked. Although many unpatched nodes continued to build on the "bad" block chain, the "good" block chain overtook it at a block height of 74691 at which point all nodes accepted the "good" blockchain as the authoritative source of Bitcoin transaction history.
The bad transaction no longer exists for people using the longest chain. Therefore, the bitcoins created by it do not exist either. While the transaction does not exist anymore, the 0.5 BTC that was consumed by it does. It appears to have come from a faucet and has not been used since.