This error (see full text below) can occur occasionally for various reasons. 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.

We suggest you clear the cookies in your browser and try again. If the issue persists, please contact us by email at webstore@illinois.edu so we can examine the situation. You may find that there is a transaction on your card account. We can determine whether this is a captured transaction or 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. 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.
 

We apologize for the inconvenience.