Be careful with the timing of your American Express payments. If your payment is just a few days late, you may still be considered as 30 days past due. According to a late payment policy study by CardHub.com, American Express counts days "past due" starting from the date your billing cycle is printed, not the date your payment is due.
Fortunately, this is just an issue of phrasing and doesn't impact late payment penalties or credit bureau reporting. It can just scare you into thinking your balance is seriously delinquent when it's only mildly delinquent.