Common proverb which may not be such good advice
I'm thinking that the old saying "Don't look a gift horse in the mouth" probably shouldn't be followed. After all, if the Trojans had done that, they may have outlasted the Greek siege and Troy might still be standing today. Food for thought.
However, one proverb that I have found to be pretty accurate is "An apple a day keeps the doctor away". When I was younger, a doctor used to cut through my property daily while walking to work. So I started hiding up in an apple tree and would peg him in the head when he passed by. Needless to say, he found another route.