I have an eCommerce site using USPS to retrieve Zip+4 info for shipping addresses. In my case, the problem was not that the USPS service failed, but rather that it succeeded and returned empty zip code and plus4 information for the requested address. Had the service failed, my site would have experienced no issue whatsoever. It is the fact that the USPS service returned a valid 'success' response that included empty zip and plus4 values that caused the problem.
Luckily in my case, this only resulted in a few orders having an empty zip code, rather than any kind of system crash. However, the empty zips caused my client considerable consternation. Of course, once I identified the cause of the problem, I added code to validate the returned zip and plus4 values from the USPS service, even when the USPS response does not indicate any error condition.