Just had an incident where the certificate did not work. They had a $20 certificate and when they checked out and tried to use the Zero Total option (since the cert covered the entire 18.95 order), this is the message they got:

Your final order total is 3.5527136788005E-15, not $0.
Please go back and select a different method of
payment.

Your order is not complete. Please review the details
of your order and click Submit Order if all the
information is correct. You may use the Back button to
make changes to your order if necessary.

Payment Method
Subtotal: $18.95
Gift Certificates: -$18.95
Total: $0.00
Paying by: Zero Total

I had to process their order manually. Everything looked fine (I've used certificates on multiple occasions to purchase new releases .. so for instance I have a $500 certificate I have used to buy a 3.99 track with just for testing).

Any ideas?