What is the test for establishing that a party relying on a presumption or of mistake or inadvertence has done?

28 The exception to this rule is where the opposing evidence is absolutely conclusive. The general rule is also subject to the qualification that where testimony of the party relying on the presumption or of his witnesses is the product of mistake or inadvertence, such testimony will not operate to dispel the presumption. (See Leonard v. Watsonville Community Hosp., supra, fn. 4, p. 517 of 47 Cal.2d p. 41 of 305 P.2d.)

29 Defendant's Nos. 41 and 42.