Examining various financial statements

After examining various financial statements that are prepared for use by internal and external stakeholders. Why are externally presented reports required to be prepared according to generally accepted accounting principles while internally presented managerial accounting reports are not? How can a misstatement in one financial statement, whether intentional or not, affect a presentation in another financial statement? Give an example of an error that occurs on one of the financial statements and the error flows through to a different financial statement.