I have a Citi AA MasterCard. My bill fell on 3rd-5th of each month until I lost my card. I paid my account in full last month via electronic transfer from my Citi checking account. A day or two after paying my statement before the due date, I reported my card lost. Because there is a 25 day cycle, I received a new date date at the end of the month for my new card. That's fine, but Citi retroactively changed my July statement due date to July 30th from August 3rd causing my already-received payment to be late.
$39 late fee AND $50+ in finance charges.
Citi agreed to take these fees off (a request was submitted), but be aware of this if you lose your card. I'm sure everyone reviews their statements, but it's still a potential problem.
Sadly, I noticed there was a problem b/c my AA miles did not match my statement balance. Admittedly, that's not very good account reconciliation, but it worked.