Can you give me an example of someone who is "regarded as" having a disability?

Yes. A woman applies for a job as a customer service representative at a department store. Her face is badly scarred from an automobile accident. The interviewer doesn’t want to give her the job, in spite of her skills and experience, because he thinks customers will be uncomfortable looking at her. She is not substantially limited in any major life activity, but the interviewer is “regarding her as” if she has a disability.