I cannot tell you how many issues I see come up in the AdWords API forum about why does a certain request work in the Google AdWords API Sandbox and not in the normal AdWords API. A Sandbox API is a test ground for developers to test out new code changes without impacting live data.

Eric Koleda from the AdWords API Team announced in a AdWords API Help thread the introduction of enhanced XML validation for the AdWords API sandbox. That means, the XML you use in the sandbox will be stricter and return more detailed errors when it does not validate. Let me quote Eric:

Forum discussion at AdWords API Help.