It may not be defined in the law, but it is defined in the dictionary. Willful is an adjective meaning "obstinately and often perversely self-willed; done deliberately; intentional."
Thus, an error (such as a clerical error) is by definition not willful. On the other hand, the ATF's behavior as described in the article does appear to be willful.
That brings to mind another slimeball excuse:
"No controlling legal authority"...