A warranty does not have to consist of writing on a piece of paper. It can be verbally created. If you are a merchant providing goods or services, you should take care to ensure that your employees are not unknowingly creating express warranties. Large companies have boilerplate disclaimers of implied and express warranties, but even those are not always successful. If the disclaimers are not conspicuous enough, or are against public policy, they will not effectively disclaim express and implied warranties. Article 2, Part 3 of the Uniform Commercial Code covers warranties in transactions among merchants. The Magnuson-Moss Warranty Act covers warranty disclosure requirements in consumer transactions.
An express warranty does not need to use the words, "warranty," or "guarantee," either. If your employee or colleague says anything about the goods or services that become part of the buyer's decision to purchase from you, an express warranty can be created.