Checkout page keeps refreshing: customer can't checkout

I received 2 emails from customers this morning saying they couldn't complete their orders. At the checkout where they enter their credit card information, they say it just keeps refreshing when they hit "continue" without telling them what the issue is. I cannot replicate the problem. I have done test orders here the office and had 2 people who have nothing to do with the company place orders from their homes with success. I don't know what's going on. I've had a big more left shopping carts than normal and I'm wondering if these people had the same problem.

can anyone help?

I was able to complete an order without error ...

Typos in the Credit Card number can cause this ...

What credit cards do you accept? What Credit Card types are failing?

I just saw your order come through. I'm not sure which type of cc they were trying to use. I accept Visa, Mastercard, Discover and Amex. Thanks for doing the test order. I'll cancel your order:)

Check in the Configuration ... Credit Cards ... that you have all those turned on ...

Yep, there are all enabled. Thanks for the help.

What gateway and shipping module are you using?

I use authorize.net but I manually process the credit cards (so the middles section of the card # gets sent to us via email).

For shipping module we just use UPS shipping (pull from UPS) and freeshipper (on a handful of products)