DohA written contract for the sale of real property included an integration clause stating that “no extrinsic evidence whatsoever may be introduced in any judicial proceedings involving this Agreement.” The referee who tried the case considered such evidence, finding for the plaintiff only to be reversed by the Court of Appeal.  Hot Rods, LLC v. Northrop Grumman Systems Corp. (2015) 242 CA4th 1166

Normally, even with an integration clause, evidence from beyond the four corners of the written agreement may be admitted to explain the meaning of ambiguous contractual language.  The Court of
Appeal held that the parties may also contract out of the rule therefore it ws error to admit extrinsic evidence to interpret the document!