The Supreme Court case of Irwin v. San Francisco Savings Union was a case that was heard in the United States Supreme Court in 1982. The case involved a dispute between a bank and a customer over the bank's refusal to honor a check that the customer had written. The customer, Irwin, had written a check for $2,000 to a third party, but the bank refused to honor the check because Irwin had insufficient funds in his account. Irwin then sued the bank for breach of contract, claiming that the bank had violated the terms of the deposit agreement by refusing to honor the check. The Supreme Court ultimately ruled in favor of the bank, finding that the bank had not breached the deposit agreement. The Court held that the bank had acted in good faith and had not acted in a manner that was contrary to the terms of the deposit agreement. The Court also held that the bank had acted reasonably in refusing to honor the check, given the fact that Irwin had insufficient funds in his account. The Court also noted that the bank had acted in accordance with the Uniform Commercial Code, which provides that a bank may refuse to honor a check if the customer does not have sufficient funds in his account. In conclusion, the Supreme Court ruled in favor of the bank in Irwin v. San Francisco Savings Union, finding that the bank had acted in good faith and had not breached the deposit agreement. The Court also held that the bank had acted reasonably in refusing to honor the check, given the fact that Irwin had insufficient funds in his account.

In the dissenting opinion of Irwin v. San Francisco Savings Union, Justice Mosk argued that the majority’s decision was wrong and should be overturned. He believed that a bank's promise to pay interest on deposits is an enforceable contract under California law, regardless of whether or not it has been approved by the state banking commissioner. Furthermore, he argued that even if there were some technical violations in this case, they did not rise to a level where enforcement would be inappropriate or unjustified. Therefore, he concluded that the plaintiff had every right to sue for breach of contract and seek damages from the defendant bank for failing to honor its obligation to pay interest on his deposit account as promised.