I cancelled my AmEx account a few years ago. I had scheduled a payment online for a couple days out when I realized that I had used the wrong bank account -- one that I was in the process of closing. I immediately called them and asked that they cancel the scheduled payment (since I could find no way to do it online). The person on the phone told me they could not do that and that once the payment was scheduled it could not be stopped.

So a couple days later, the payment was refused due to insufficient funds and I was dinged for it.