Correlations can never always be estimated correctly. They'll work for a while, but as soon as the underlying assets stop acting according to the 'rules' upon

Correlations can never always be estimated correctly. They'll work for a while, but as soon as the underlying assets stop acting according to the 'rules' upon which the original correlations were calculated, disaster strikes. That turning point always occurs--it's not a matter of 'if' but 'when' and 'how severe'.