Only thing I can think of is a US passport also requires a tourist card entering Panama. But they will sell you one right at the border, for $5 I think. They sometimes run out of those at the crossing at Guabito, but going through on the Pacific side I wouldn't think would be an issue, since it's so much busier and they likely have a lot of those cards on hand.

Another issue we had crossing at Guabito is they required proof that we had reservations out of the country. If you're taking a boat or plane over to Columbia or Ecuador, have documentation of that with you. They made us buy bus tickets that we'd never use because we had none of that.