Boy, this question is loaded…in today’s information sharing age, there is more info than ever being told to more golfers than ever before. While it’s great to share info, it’s often difficult for amateurs to filter out the gold from the noise. And the reality is there is plenty of noise that isn’t very good or applicable.
To answer the question though, the worst advice is probably when someone is told to swing a certain way because “everyone who’s good does it that way.” While the person providing the info probably means well, the reality is when you look at the best players in the world, everybody is doing things different. Even at the club level no two “good” scratch players are doing things the same. One way is not going to work for most in any facet of the game.
Instead of this advice, the amateur player would be best served if he or she focused on problem solving his or her own game/swing and finding solutions that work well for them. Working through this problem-solving process with a trusted professional coach is probably best- someone who can filter advice and assure it’s relevant for you and your game.