If someone is deemed to be deceptive on 1 relevant question but is deemed truthful on the rest of the relevant questions, does that person necessarily fail the polygraph or will there be a retest?

neogeo,

Anxietyguy is correct: being deemed deceptive on one relevant question means failing the polygraph. I would just note, however, that with two federal agencies in particular (CIA and NSA), it is routine for applicants to be initially accused either of outright deception or simply "having problems" with regard to one or more relevant questions, and (unless a disqualifying admission has been made) to be re-scheduled for one or more "re-tests."