Employers that tell an applicant she's hired and then yank the offer should be prepared to give a legitimate, business-related reason. Carefully document the reason in case the applicant later sues. You will need a solid explanation to persuade a court that someone you once believed was well-qualified really wasn't.
Recent case: Priscilla worked for the Federal Emergency Management Agency in Chicago as a disaster reservist. FEMA fired her when she refused to accept deployments when called up. She filed an internal discrimination complaint, which was unsuccessful.
About a year later, she applied for a similar job with FEMA in the Philadelphia area. She was offered the job.
Then, before Priscilla's first day on the job, the Philadelphia office rescinded the job offer when it learned the Chicago office had fired her for refusing deployments. The Philadelphia office documented...