What one person has the ability to make you feel unsure about facts you ordinarily wouldn't question?

Close trusted family would be my first suggestion. For example I completely trust my brother and if he was to turn around and make a statement that normally I'd p off as crazy it would certainly make me stop and think twice. I guess its that level of trust and respect. You certainly don't put as must trust in the word of a stranger compared to the word of a close friend or respected family member.