This error (see full text below) can occur when your WebStore purchase with Credit Card is not completed properly. This typically happens during times of heavy traffic in the shop.  The exact cause is unclear at this time, but WebStore is working to find the cause and remedy it.

As suggested in the message, please contact us webstore@illinois.edu so we can examine the situation. You may find that there is a transaction on your card account. We can determine if this is a captured transaction or if it is just an authorization. Once we examine your purchase attempt, we can assist in completing the transaction.
 
Full error text:
"Well, this is embarrassing...
 An error may have occurred, and your request may not have been completed. If you were trying to make a purchase, please click on the back button and check your Order History before trying your purchase again. Do not hit submit a second time. We apologize for the inconvenience.