Email received from American Express:
We sincerely apologize for any inconvenience you may have experienced over the weekend when using your Card to make a purchase or withdraw money at an ATM. We experienced high transaction volumes
and some intermittent disruption to our services. Because of heavy call volumes, we were unable to provide you with the level of service that you should expect.
We are working quickly to resolve any lingering issues, including correcting any unexpected pending transactions that you may see on your account.
That last bit is perhaps a little scary, but I noticed no such transactions, and went back to check just to make sure.