Currently, you must use the Fiscal Year Configuration to close a fiscal period during the year.  While in there, if you accidentally change an End Date (and don't notice it), the system will change that End Date and the Beginning Date of the next period.  It does not give you a warning that dates have changed.  Since you were in there to close a period, you save the record, completely unaware that the fiscal year period structure has changed.   Once transactions are posted to that period, you will be unable to correct the date in Fiscal Year Configuration and the transactions will be posted to incorrect period.