Question from a civilian about enlisted/officer ranks

I'm fairly certain this question applies to American and Commonwealth armies. Not sure about others.

It is frequently said that a newborn second lieutenant should listen to his sergeant's advice, and follow it. The 2LT outranks the SGT, but the SGT has far more practical experience.

Are there any circumstances where a 2LT reasonably should overrule his sergeant? If there aren't, then why doesn't the rank structure reflect that?