Trustmark Insurance Company (Trustmark) (plaintiff) maintained bank accounts at Bank One, Arizona, NA (Bank One) (defendant). In May 1995, Trustmark set a letter to Bank One. The letter instructed Bank One to: (1) retain a daily balance of $10,000 in the account and (2) automatically transfer funds to another account at the Harris Bank whenever the Bank One account reached a balance of $110,000 or more. Bank One did not honor the request, and Trustmark’s account contained more than $110,000 from September 1996 to December 1997. In December 1997, Bank One notified Trustmark that the account contained $19,230,099.80. All but $10,000 of that money was transferred to the account with Harris Bank. Trustmark sued Bank One under Article 4A of the Uniform Commercial Code (UCC), claiming a loss of more than $500,000 in interest based on Bank One’s failure to transfer the funds. The jury found in favor of Trustmark, and Bank One appealed to the Court of Appeals of Arizona.