In addition to what Hauling just said. If you have the receipt, keep it and put it in with your Citi paperwork. Make notes of the dates and times you spoke to them about the issue and if possible the names of the CSRs you spoke with. If it comes up down the road and they try to state is was approved on a different date, you have the receipt and can prove their error if it wasn't on a subsequent statement (leaving a gap). CCCs will almost never admit an error on their part unless you have some kind of proof, then they'll often treat it with "oh, ok, no problem, we'll fix that right away for you".
Oddly enough, I've overpaid Cap 1 through their website ("pay my bill") link and it hasn't been an issue, I wonder if Citi works the same way now? I've read some allow you to overpay your balance by up to 10% but I don't know if it's fact or not.
