This article discusses the California Supreme Court case Sterling v. Taylor which held that extrinsic evidence may be admitted to support a claim that the Statute of Frauds has been met. When a writing is not sufficient, the attorney must not only decide whether there is sufficient supporting evidence to get past the Statute of Frauds, but also whether that evidence is sufficiently consistent with the writing.
Bernhardt, Roger, "The Statute of Frauds" (2007). Publications. 394.