Is the unauthorized presence of an alternate juror during jury deliberations reversible error?

[36 Cal.App.3d 409] California is not alone in holding that the unauthorized presence of an alternate during jury deliberations is reversible error. In United States v. Beasley, 464 F.2d 468 (10th Cir. 1972), the alternate juror went to the jury room with the regular jurors, voted to select a foreman and then voted to go to lunch. The alternate was with the jury about 20 minutes before the court removed her. A motion was made for a mistrial and a hearing was held to determine the extent the alternate had participated in the deliberations. The trial court found no prejudice and denied the motion. On appeal, the conviction was reversed with directions to grant a mistrial, and the reviewing court stated: