about the overflow incident:
"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.[7] "
https://en.bitcoin.it/wiki/Value_overflow_incidentdoes that mean we have 21m+0.5 bitcoin?
No. Look at the transaction in a block explorer. It has one 235,5 BTC input and two (0,5 BTC and 235,0 BTC) outputs. So, unless there were 21m+0,5 BTC before that transaction, this isn't the case afterwards, either.