Some of you will say, in this day and age, I can wear whatever I want, and you're quite right. At the same time, you can breathe asbestos and drink water from lead pipes. Just because you can do something does not mean you should.
The first thing you should never wear as a gentleman are sandles. They look particularly bad when combined with socks. Unless you are at a pool or the beach where they are appropriate, they will make you look immature and unintelligent. The same goes for flip flops. What you want are a nice pair of oxfords or for a more relaxed look a pair of loafers maybe.
The second thing you should never wear are cargo shorts. They just make you look disorganized. The pockets are often uneven and send a subconscious signal to others that you are just not very well put together.
The third thing you should never do are wear shoes without socks, or with socks that are too short. No one wants to see your hairy calves, and even if you do shave them, it's just not appropriate in a business or formal setting.
There is no pulling off a man bun if you want people to respect you. Period.
Square toed shoes: They are ugly and tell people that you don't care about your appearance. It's always best to go with a round toe or a chiseled toe. They just look more elegant.
Sports sunglasses: something you should never wear with a formal wardrobe. The type of thing you see people wearing out in the woods or while biking, often curved around the head and reflective or rainbow colored. it is better to go with a pair of classic sunglasses that have stood the test of time. Try a nice pair of aviators, wayfarers, or more square framed glasses instead.
Jewelry: A fancy bracelet or necklace is nice, but you don't want to be wearing big, heavy, gaudy gold chains. Also, aside from a wedding or engagement ring you don't want to have more than one ring on each hand. doing so is excessive and looks pompous.
Jerseys: these should only ever be worn at sporting events to support your team or at your Superbowl Sunday game day party. Any other use is wholly inappropriate and make you look immature. The same goes for t-shirts with slogans on them. They make you look like a 13 yer old kid who wants to express himself but doesn't know how. Express yourself with your words.
Backpacks should only ever be worn by schoolchildren or if you're hiking in the wilderness or climbing a mountain. If you absolutely must have a bag, try a nice leather satchel or a briefcase instead.
Sports or digital watches. If you're wearing a suit you'll want to go with a more classic analog watch that portrays an air of experience and sophistication.
Ties: there are a lot of things you can do wrong with a tie.
Wearing a tie with a shirt that is not buttoned all the way up. It sends the message that you don't care about what you look like. Wear a tie properly or not at all.
Tie knots that are too big or ties too long: again, a big no-no. It makes you look like a child. A tie should never hang below your belt or stick out below a vest.
Wearing neckties to black tie events is just plain wrong, and makes you look like you don't know what you're doing.
Suits: There are a lot of different suits out there. Some of them look nice, others do not. If you want to look good you'll want something that fits properly. Arm holes should be the proper size and should fit snugly around the shoulders. They should not be too long. You don't want to look like a little kid who got into daddy's closet and decided to play dress up.
Sagging pants: for those of you who don't know. The whole sagging pants thing started in prisons as a way of letting the other inmates know that you were "available". If this is not the message you want to send, then wear your pants up on your waist properly where they belong.